Epidemiology (Study Designs, Bias, Systematic Review, Measures of Association) MCQs

Community Medicine (PSM) · 276 free questions with answers & explanations.

1. A researcher studies a group of 500 textile workers exposed to cotton dust and 500 unexposed workers over 10 years to determine incidence of byssinosis. The relative risk for developing byssinosis in exposed workers is 6.0. Which type of study design is this?
2. In a case-control study on oral cancer, 80 out of 100 cases and 40 out of 100 controls were found to be tobacco chewers. What is the odds ratio for tobacco chewing and oral cancer?
3. A systematic review pools data from multiple randomized controlled trials to assess the efficacy of a new antimalarial drug. Which statistical method is used to combine quantitative results from these individual studies?
4. In a cross-sectional survey of blood pressure, individuals with severe hypertension who died before the survey could be enrolled are missed. Which type of bias does this best represent?
5. Attributable risk percent (AR%) for smoking and lung cancer is 90%. This figure is most useful for which of the following purposes?
6. A cohort study follows 2,000 smokers and 2,000 non-smokers for 10 years. At the end, 80 smokers and 20 non-smokers develop lung cancer. The Attributable Risk Percent (ARP) in smokers is:
7. In a case-control study on cervical cancer, 60 of 100 cases and 30 of 100 controls were exposed to HPV. The Odds Ratio is:
8. Berkson's bias is a type of selection bias unique to which study design?
9. A meta-analysis of 10 RCTs shows a pooled RR of 0.75 (95% CI: 0.60–0.95) for a drug reducing MI. The I² statistic is 68%. What is the correct interpretation?
10. The Bradford Hill criterion that CANNOT be adequately assessed using ecological study data is:
11. A cohort study examining lung cancer risk among asbestos workers finds a Relative Risk of 3.8 (95% CI 2.1–6.9). When researchers calculate the Attributable Risk Percent (ARP) among the exposed, which formula correctly represents this measure?
12. In a systematic review, significant heterogeneity is detected across included studies (I² = 78%, p < 0.001). Which meta-analytic model is most appropriate to use, and what is the underlying rationale?
13. A researcher compares smoking rates between cases (lung cancer patients) and controls recruited from the same hospital's orthopedic ward. The odds ratio for smoking is found to be 1.2 (95% CI 0.9–1.6), lower than the known true association. The most likely explanation is:
14. A study of a new rapid diagnostic test for malaria in a high-transmission district finds sensitivity 92%, specificity 85%. The disease prevalence in this population is 40%. What is the Positive Predictive Value (PPV) of this test?
15. In an ecological study evaluating sodium intake and hypertension prevalence across 20 countries, researchers find a strong positive correlation (r = 0.85). A researcher then concludes that individuals who eat more sodium have higher risk of hypertension. This logical error is called:
16. A randomized controlled trial compares two antihypertensive regimens. After analysis, the researchers perform multiple sub-group comparisons and find a significant benefit in a subgroup of elderly diabetics (p = 0.03). This finding is best characterized as:
17. A systematic review of 12 randomized controlled trials on a new antihypertensive drug shows a pooled relative risk reduction of 25%. However, the Cochran Q test yields p = 0.003 and I² = 78%. The most appropriate interpretation of these findings is:
18. In a cohort study examining the association between pesticide exposure and Parkinson's disease, the risk ratio is 2.4 and the 95% confidence interval is (0.9, 6.3). Which statement best describes this result?
19. Researchers conducting a case-control study on oral cancer and tobacco use note that cases recalled tobacco exposure more thoroughly than controls because they had extensively discussed their habits with physicians after diagnosis. This is best described as:
20. A cross-sectional study finds that regular aspirin users have a lower prevalence of cardiovascular disease compared to non-users (prevalence ratio 0.65). An epidemiologist argues this estimate may be biased because aspirin users who developed serious cardiovascular disease are less likely to be alive and thus not enrolled. This phenomenon is:
21. In the Bradford Hill criteria for causation, the criterion that an observed association makes biological sense given current knowledge of pathophysiology is called:
22. A researcher conducts a case-control study to assess the association between pesticide exposure and non-Hodgkin lymphoma. Cases recall their past exposure more vividly due to their diagnosis. This is an example of which type of bias?
23. In a cohort study following 1000 smokers and 1000 non-smokers over 20 years, 200 smokers and 50 non-smokers develop COPD. The attributable risk percent among exposed is closest to:
24. A researcher designs a study where participants with and without a risk factor are observed over time. Midway through, a new treatment becomes available that prevents the disease and is preferentially given to the high-risk group. This introduces which type of bias?
25. In a meta-analysis, the I² statistic is reported as 78%. The most appropriate interpretation is:
26. Which of the following study designs is LEAST prone to incidence-prevalence bias (Neyman's bias)?
27. A case-control study comparing 200 cases of bladder cancer with 200 controls finds an odds ratio of 3.2 (95% CI: 1.8–5.7) for cigarette smoking. A systematic review of 12 similar studies reports a pooled OR of 2.9 (95% CI: 2.4–3.5) with I² = 18%. Which statement about the systematic review finding is MOST accurate?
28. In a prospective cohort study of 5000 smokers and 5000 non-smokers followed for 10 years, 150 smokers and 30 non-smokers develop lung cancer. The attributable risk percent (ARP) in smokers is:
29. A researcher conducts a cross-sectional study to determine the association between obesity and hypertension. Subjects are recruited from outpatient clinics. More obese individuals attend the clinic because they also have diabetes requiring frequent follow-up. This is an example of:
30. In a randomized controlled trial, allocation concealment is BEST achieved by:
31. The Number Needed to Harm (NNH) for a drug causing liver toxicity is 200. This means:
32. A researcher conducts a cohort study comparing smokers and non-smokers for lung cancer incidence over 20 years. Loss to follow-up occurs disproportionately among the sickest smokers who died early. This is best described as:
33. In a meta-analysis, the I² statistic is reported as 82%. The forest plot shows wide confidence intervals for individual studies with several overlapping the null. The most appropriate next step is:
34. A case-control study reports an odds ratio (OR) of 3.5 with 95% CI of 1.1–11.2 for the association between pesticide exposure and Parkinson's disease. The wide confidence interval most directly reflects:
35. Berkson's bias most commonly threatens the validity of which study design?
36. The Population Attributable Risk Percent (PAR%) for hypertension as a cause of stroke in a community is 55%. This implies that:
37. In a prospective cohort study examining the relationship between occupational asbestos exposure and mesothelioma, 5,000 exposed workers were followed for 20 years. Of 200 who developed mesothelioma, 180 were exposed. Among 10,000 unexposed controls, 40 developed mesothelioma. The Attributable Risk Percent (Etiologic Fraction) in the exposed group is:
38. A systematic review pooling five randomized controlled trials on a new antihypertensive shows an I² statistic of 75%. The most appropriate next step in the meta-analysis is:
39. A case-control study on oral contraceptive use and hepatic adenoma is conducted. Cases are recruited from hepatology clinics and controls from the same hospital's orthopedic wards. Women with known liver disease are excluded from both groups. This control selection is most likely to introduce which type of bias?
40. In a randomized controlled trial comparing Drug A vs placebo for prevention of MI, the absolute risk reduction (ARR) is 2% and the relative risk reduction (RRR) is 40%. The Number Needed to Treat (NNT) is:
41. A study compares regional rates of salt consumption with hypertension prevalence across 20 Indian states and finds a strong positive correlation (r = 0.82). A policy maker concludes that individuals who consume more salt have higher rates of hypertension. This reasoning exemplifies:
42. A case-control study examining the association between dietary fat intake and pancreatic cancer recruits hospital patients with recently diagnosed pancreatic cancer and matches them with orthopaedic surgery patients. The controls are more likely than the general population to have low-fat diets because orthopaedic patients are advised to reduce weight before surgery. This constitutes which type of bias?
43. In a meta-analysis of trials testing a new antihypertensive, a funnel plot shows asymmetry with a conspicuous absence of small negative studies in the lower-left region. The MOST likely explanation is:
44. A prospective cohort study follows 5000 smokers and 5000 non-smokers for 10 years. Among smokers 200 develop lung cancer (incidence rate 40/1000); among non-smokers 10 develop lung cancer (incidence rate 2/1000). The Attributable Risk Percent (ARP) among the exposed is:
45. A researcher conducts a cross-sectional survey to estimate prevalence of hypertension. She finds that individuals with hypertension are over-represented because they make more clinic visits and are thus more likely to be included in the sample. This is best described as:
46. In a randomised controlled trial, the concept of 'intention-to-treat' analysis is used primarily to:
47. A study measures the association between coffee consumption and atrial fibrillation. Researchers find an OR of 1.4 (95% CI 1.1–1.8). A reviewer notes that coffee consumption is correlated with smoking, which is the true risk factor. Adjustment for smoking attenuates the OR to 1.0. The original finding illustrates:
48. An investigator designs a study to estimate the incidence of type 2 diabetes in an urban slum over 5 years by repeatedly examining the same cohort annually. At baseline, all participants are diabetes-free. What is the key advantage of this design over a case-control study for this purpose?
49. A Mendelian randomization study uses genetic variants as instrumental variables to assess the causal effect of LDL cholesterol on coronary heart disease. For an instrumental variable to be valid, which of the following assumptions must be satisfied?
50. A researcher conducts a prospective cohort study and finds Relative Risk (RR) = 0.6 with 95% CI (0.4–0.9) for a dietary intervention on cardiovascular disease. Which statement best interprets this finding?
51. In a case-control study on lung cancer, 70% of cases and 40% of controls smoked. The Odds Ratio (OR) is calculated. Compared to a cohort study with the same population, the OR will approximate RR only when:
52. A systematic review reports I² = 78% across 12 included studies. Which statement about this finding is MOST accurate?
53. A researcher studies association between arsenic exposure and bladder cancer using stored blood samples from a cohort enrolled 10 years ago. This study design is best described as:
54. In a cross-sectional study, prevalence of hypertension was higher among those with longer sleep duration. However, this association disappeared after adjusting for age. This is an example of:
55. The Number Needed to Harm (NNH) for a drug causing hepatotoxicity is 150 over 2 years. What does this mean?
56. A case-control study examining the association between pesticide exposure and non-Hodgkin lymphoma enrolls patients from a tertiary oncology centre and selects controls from the same hospital's orthopaedic wards. Both cases and controls are asked to recall pesticide exposure over the preceding 10 years. Which bias MOST threatens the internal validity of this study?
57. In a randomized controlled trial of a new anti-hypertensive, participants in the treatment arm also spontaneously reduce salt intake because they know they are being treated. The observed blood pressure reduction in the treatment arm is thus larger than the drug effect alone. This phenomenon is BEST described as:
58. A meta-analysis of 12 trials shows a funnel plot that is markedly asymmetric, with small studies clustered on one side. The MOST likely explanation for this pattern is:
59. In a prospective cohort study, the incidence rate among exposed individuals is 45 per 1000 person-years and among unexposed is 15 per 1000 person-years. What is the attributable risk percent (ARP) among the exposed?
60. A cross-sectional survey finds a high prevalence of rheumatoid arthritis (RA) among users of a specific herbal supplement compared to non-users. A researcher concludes the supplement causes RA. The MOST important flaw in this inference is:
61. Which of the following study designs would provide the STRONGEST evidence regarding a rare adverse drug effect that occurs 1 in 10,000 exposures, when following up large cohorts is logistically impossible?
62. In a cohort study, confounding by indication is a major concern. A researcher uses propensity score matching to address this. Propensity score is defined as:
63. A systematic review of observational studies uses the GRADE framework to rate the quality of evidence. Observational studies start at which initial evidence level, and which factor can UPGRADE that rating?
64. A case-control study investigates the association between dietary fat intake and colorectal cancer. Participants are asked to recall their diet from 5 years ago. The OR comes out as 2.8, but a subsequent cohort study yields an RR of 1.4. The most likely explanation for the discrepancy is:
65. In a randomized controlled trial comparing two antihypertensive drugs, analysis is done only on patients who completed the full 12-month follow-up, excluding dropouts. This analytical approach is termed:
66. A meta-analysis of 15 studies on exercise and type 2 diabetes reports a pooled RR of 0.72 (95% CI: 0.68–0.76), I² = 78%, and a funnel plot with visible asymmetry. The most appropriate next step in interpreting this meta-analysis is:
67. In an ecological study examining per-capita sugar consumption and diabetes prevalence across 30 nations, a strong positive correlation (r = 0.85) is found. A public health official uses this to infer that individuals who consume more sugar develop diabetes. This logical fallacy is called:
68. A prospective cohort study of coal miners follows two groups: those with >10 years of mining exposure and those with <2 years. The incidence of pneumoconiosis is 45/1000 person-years vs. 3/1000 person-years respectively. The attributable risk percent (AR%) in the exposed group is approximately:
69. A researcher conducts a cross-sectional study of hypertension prevalence and notices that patients with hypertension also have higher rates of a certain genetic variant. When age is controlled for, the association disappears because older age independently causes both hypertension and the variant to increase. This is an example of:
70. A study recruits all patients attending a cardiology OPD. Controls are selected from the same OPD but have respiratory disease. The study finds an inflated OR for the association between smoking and heart disease. The specific bias operating here is:
71. In assessing the strength of a causal association using Bradford Hill criteria, the criterion that is considered MOST fundamental (necessary but not sufficient) for causation is:
72. A case-control study is conducted to examine the association between dietary fat intake and colorectal cancer. Cases are recruited from hospital oncology wards, while controls are patients admitted for non-gastrointestinal, non-cancer conditions. The odds ratio (OR) for high fat intake among cases vs controls is 1.8 (95% CI: 1.2–2.7). The investigators note that dietary recall over the past year is likely influenced by awareness of cancer diagnosis. Which specific bias best explains this distortion?
73. In a prospective cohort study comparing smokers (n=2,000) and non-smokers (n=5,000) over 10 years, 80 smokers and 40 non-smokers developed lung cancer. What is the population attributable risk percent (PAR%) assuming smokers constitute 20% of the general population? (Incidence in smokers = 40/1000; Incidence in non-smokers = 8/1000; Overall population incidence = 14.4/1000)
74. A systematic review of randomized controlled trials reports that a new antihypertensive drug reduces cardiovascular events. The I² statistic is calculated as 78%. Which conclusion is MOST appropriate regarding this meta-analysis?
75. A researcher wants to study the causal pathway between childhood malnutrition, adult poverty, and adult cardiovascular disease. She finds that adult poverty is associated with both malnutrition history and cardiovascular disease, but is itself a consequence of early childhood disadvantage. In the directed acyclic graph (DAG) framework, adult poverty in this scenario is BEST described as:
76. In a cross-sectional seroprevalence survey of dengue IgG antibodies, a significantly higher prevalence is found in elderly subjects than in young adults. A virologist suggests this reflects higher past exposure in the elderly cohort, not aging per se. This difference between age-effect and cohort-effect is best untangled by:
77. In a prospective cohort study examining the association between occupational silica dust exposure and lung fibrosis, workers who developed severe respiratory symptoms were more likely to leave their job and thus were lost to follow-up. This scenario best illustrates which type of bias?
78. A meta-analysis of randomised controlled trials evaluating a new antihypertensive drug finds high heterogeneity (I² = 78%). The most appropriate statistical model and the most appropriate interpretation are:
79. In a case-control study, 200 cases of hepatocellular carcinoma and 400 controls were enrolled. Of the cases, 80 had hepatitis B; of controls, 40 had hepatitis B. The odds ratio is closest to:
80. A researcher investigates whether low birth weight causes childhood asthma. She enrolls children with asthma (cases) and without asthma (controls) and then asks mothers to recall their child's birth weight. Cases' mothers are more likely to over-report problems at birth. This best describes:
81. A cross-sectional study conducted in a community finds a higher prevalence of diabetes among those with low physical activity compared with those who exercise regularly. The major threat to causal inference in this study design is:
82. In an ecological study, countries with higher fat intake show higher rates of breast cancer. A researcher concludes that dietary fat causes breast cancer. The specific logical flaw in this conclusion is termed:
83. A randomised controlled trial of a new drug uses allocation concealment but fails to blind participants or outcome assessors. The most important consequence of this specific methodological gap is:
84. Which of the following study designs provides the strongest evidence that a new screening test reduces disease-specific mortality at the population level?
85. A case-control study is conducted to evaluate the association between pesticide exposure and non-Hodgkin lymphoma. Controls are selected from the same hospitals where cases are admitted. The odds ratio is estimated as 3.4. Which specific bias is MOST likely to have inflated this estimate?
86. A 10-year prospective cohort study finds that among 5,000 exposed workers and 5,000 unexposed workers, 200 and 100 cases of lung cancer develop, respectively. What is the Population Attributable Risk Percent (PAR%) if the prevalence of exposure in the total population is 30%?
87. In a randomized controlled trial comparing two antihypertensive drugs, randomization fails to balance a major confounder (age) between groups despite adequate sample size. Which of the following BEST describes this phenomenon?
88. A systematic review pools results from 8 studies evaluating a dietary intervention. The I² statistic is 72%. Which interpretation is MOST appropriate?
89. A cross-sectional survey finds higher rates of diabetes among individuals who exercise regularly compared to those who do not. The most plausible explanation for this counterintuitive finding is:
90. An investigator uses the number of incident cases of tuberculosis over 5 years divided by the total person-time contributed by all study participants. This measure is BEST described as:
91. A drug company-sponsored RCT shows a drug reduces cardiovascular events by 2% in absolute terms (from 4% to 2%), but the relative risk reduction is reported as 50%. The primary critique of presenting only relative risk reduction is that it:
92. In an ecologic study, a positive correlation is found between national fat consumption and breast cancer mortality. Which fundamental limitation of ecologic studies MOST directly prevents causal inference from this finding?
93. A case-control study investigating pesticide exposure and non-Hodgkin lymphoma enrolls cases from a cancer registry and controls from the same hospital's orthopaedic ward. The main validity concern regarding this design is:
94. In a 10-year prospective cohort study, the incidence of myocardial infarction (MI) in smokers is 12 per 1000 person-years and in non-smokers is 4 per 1000 person-years. The population prevalence of smoking is 30%. The Population Attributable Risk Percent (PAR%) for smoking in MI is closest to:
95. A systematic review pooling five RCTs finds a significant pooled OR of 0.65 (95% CI: 0.50–0.85; I² = 68%) for a new antihypertensive. The high I² value indicates:
96. A researcher conducts a cross-sectional survey and finds a significant association between dietary fat intake and the presence of gallstones on ultrasound. The primary limitation of this design for establishing causality is:
97. A disease surveillance system classifies all hospital-admitted cases as 'confirmed'. When disease prevalence in the community is low (1%) but hospitalization is more common among severe cases, the calculated hospital-based prevalence will be:
98. In a matched case-control study, 200 case-control pairs are enrolled. For a discordant pair where the case was exposed and the control was unexposed, the appropriate statistic used in the Mantel-Haenszel analysis is:
99. A new biomarker for colorectal cancer has sensitivity 85% and specificity 80%. In a population with 2% prevalence, which calculation correctly estimates the Positive Predictive Value (PPV)?
100. The 'Hawthorne effect' in clinical trials refers to:
101. In a cohort study comparing smokers and non-smokers for lung cancer risk, the relative risk was 12. In a subsequent case-control study on the same population, the odds ratio was 14.4. This difference is BEST explained by the fact that:
102. A nested case-control study was conducted within a prospective cohort of factory workers. Cases of bladder cancer (n=80) were identified and 320 controls were selected from the same cohort. Blood samples collected at baseline were analyzed. The PRIMARY advantage of this design over a conventional case-control study is:
103. A systematic review meta-analysis on antihypertensive therapy shows significant heterogeneity (I² = 78%, p < 0.001). The MOST appropriate statistical model to pool the effect estimates is:
104. In a randomized controlled trial of a new antidiabetic drug, patients in the intervention group received drug plus dietary counselling, while controls received placebo only. At end of study, HbA1c reduction was significantly greater in the treatment group. This trial suffers from:
105. Berkson's bias is a systematic error specific to hospital-based case-control studies. It arises because:
106. A researcher conducts a randomized controlled trial comparing two antihypertensive drugs. After randomization, it is discovered that the allocation sequence was predictable because sealed envelopes were used but they were held up to light to view the assignment before enrollment. This flaw is best described as:
107. In a cohort study examining alcohol and liver cirrhosis, 400 heavy drinkers and 400 non-drinkers are followed for 10 years. Cirrhosis develops in 80 heavy drinkers and 20 non-drinkers. The attributable risk percent in the exposed group (AR%) is:
108. A systematic review finds five RCTs on a new drug for hypertension. The Cochrane I² statistic is 78%. The most appropriate next step in the meta-analysis is:
109. A case-control study on mobile phone use and glioma enrolls patients from a neurosurgery ward. Controls are selected from the orthopedic ward of the same hospital. Which type of bias is most likely introduced?
110. A cross-sectional survey finds a strong association between regular aspirin use and reduced prevalence of colorectal polyps. A cardiologist notes that patients on aspirin are more frequently screened via colonoscopy. This phenomenon—where treatment-associated surveillance detects more disease—best exemplifies:
111. A nested case-control study is conducted within a large prospective cohort. Compared to a conventional case-control study, the key methodological advantage of the nested design is:
112. A researcher conducts a cohort study to examine the association between smoking and lung cancer. At baseline, smokers who already have undiagnosed early lung cancer are more likely to quit smoking before enrolment, making them appear as non-smokers. This introduces which specific bias?
113. In a randomised controlled trial evaluating a new antihypertensive, patients who feel better from placebo effects are more likely to continue attending follow-up visits. Compared to the treatment arm, higher dropout occurs in truly uncontrolled patients in the control group. The resulting estimate of treatment efficacy is most likely to be:
114. A systematic review pools four case-control studies. When Mantel-Haenszel pooled OR is calculated assuming a common effect, the I² statistic is 72%. The most appropriate next step is:
115. In a cross-sectional survey, the prevalence of hypertension is compared between rural and urban populations after age-standardisation using the same reference population. The technique of age-standardisation is used to control for which type of variable?
116. A study compares the incidence of myocardial infarction (MI) in patients taking low-dose aspirin versus those not taking it. Patients already taking aspirin for an existing cardiovascular condition have a higher baseline risk. The resulting spurious apparent harm from aspirin is an example of:
117. In a funnel plot from a meta-analysis, studies with small sample sizes and large effect sizes cluster to one side of the plot, leaving an asymmetric appearance. This pattern most directly suggests:
118. A case-control study of oral cancer recruits hospital controls from dentistry outpatient clinics. Because dentistry patients are more health conscious and less likely to use tobacco, this selection of controls introduces:
119. In a cohort study of 5,000 smokers and 5,000 non-smokers followed for 10 years, 200 smokers and 40 non-smokers developed lung cancer. The attributable risk percent (AR%) in the exposed group is:
120. A researcher conducting a case-control study on oral cancer finds that cases are more likely to recall and report previous tobacco use than controls. This introduces which type of bias?
121. In a cross-sectional study on hypertension prevalence, the Mantel-Haenszel method is used to control for age confounding. The Mantel-Haenszel odds ratio is 2.1. The crude odds ratio was 3.8. This finding suggests that age was acting as a:
122. Which of the following study designs provides the highest level of evidence for establishing causation in clinical epidemiology?
123. In the Bradford Hill criteria, 'biological gradient' refers to:
124. An ecological study finds a strong positive correlation (r = 0.85) between per capita fat intake and breast cancer rates across 20 countries. A subsequent individual-level cohort study shows no association. This discrepancy is best explained by:
125. In a meta-analysis, heterogeneity between studies is measured by the I² statistic. An I² of 75% indicates:
126. The 'funnel plot' in systematic reviews is used to detect:
127. In an epidemiological study, the term 'incubation period' differs from 'latent period' in that:
128. A community randomized trial (cluster RCT) differs from an individual RCT primarily in that:
129. An observational study finds that drinking green tea (exposure) is inversely associated with gastric cancer (OR = 0.65, 95% CI 0.48–0.88). Which aspect of Hill's criteria is BEST supported by this finding alone?
130. A vaccine with 70% efficacy is deployed in a population where baseline attack rate (without vaccine) is 20%. The attack rate in vaccinated individuals will be approximately:
131. In a matched case-control study, Odds Ratio is calculated using:
132. The 'Hawthorne effect' in clinical and community studies refers to:
133. In a case-control study, an investigator selects hospital controls from the same wards as cases. Some controls are admitted for conditions causally linked to the exposure of interest (e.g., COPD patients as controls in a lung cancer study examining smoking). This introduces:
134. A cohort study finds an Attributable Risk (AR) of 15 per 1000 person-years and an incidence in the unexposed group of 5 per 1000 person-years. The Relative Risk is:
135. A researcher uses propensity score matching in an observational study comparing two surgical techniques. The primary purpose of propensity score matching is to:
136. In a Directed Acyclic Graph (DAG) used in epidemiology, a 'collider' on the path between exposure and outcome, when conditioned upon, will:
137. The Population Attributable Risk Percent (PAR%) for smoking in lung cancer is 80%. The most accurate interpretation is:
138. A study examining mobile phone use and glioma recruits cases from neurosurgery units and controls from orthopaedic wards with fractures — an exposure (phone use) completely unrelated to the control condition. Which type of validity concern is LEAST addressed by this design?
139. A researcher notices that patients who survive a disease are more likely to be enrolled in a long-term follow-up study than those who die early. This is an example of:
140. In a randomized controlled trial, the intervention and control groups differ at baseline in age despite randomization. The most appropriate method to handle this imbalance is:
141. A study finds that both physical activity and fruit consumption are inversely associated with cardiovascular disease, and physically active people also tend to eat more fruit. This introduces a potential:
142. A case-control study calculates an Odds Ratio of 3.2 for the association between alcohol use and oesophageal cancer. The Population Attributable Risk Percent (PAR%) in a population with 30% alcohol exposure prevalence is closest to:
143. A drug company funds a trial comparing their new drug versus a subtherapeutic dose of the competitor's drug. The bias introduced by using an unfair comparator is called:
144. Which of the following study designs provides the highest level of evidence for assessing the effectiveness of a vaccine in a real-world population after licensure?
145. A researcher studying the association between alcohol consumption and oropharyngeal cancer notices that smokers tend to drink more alcohol. When controlling for smoking, the association between alcohol and cancer weakens substantially. This phenomenon is BEST described as:
146. In a case-control study of bladder cancer and artificial sweetener exposure, cases are recruited from urology clinics while controls are recruited from orthopaedics clinics in the same hospital. Sweetener use happens to be higher among orthopaedic patients than in the general population. This leads to:
147. Which of the following Bradford Hill criteria is the ONLY one that, if present, is considered SUFFICIENT on its own to infer causality?
148. In a prospective cohort study, the incidence rate of cardiovascular disease in smokers is 12 per 1000 person-years and in non-smokers is 4 per 1000 person-years. What is the Attributable Risk Percent (AR%) in the exposed group?
149. A randomised controlled trial uses allocation concealment but does not blind participants or outcome assessors. Which type of bias is most likely to be introduced?
150. An ecological study examines the correlation between national per-capita fat consumption and national breast cancer rates and finds r = 0.85. A researcher concludes that high-fat diets cause breast cancer in individuals. This reasoning commits which fallacy?
151. A case-control study of a rare occupational cancer reports an odds ratio (OR) of 3.2. In this scenario, the OR is considered a good approximation of the relative risk (RR) primarily because:
152. Which of the following study designs provides the HIGHEST level of evidence in the hierarchy of evidence-based medicine?
153. An investigator detects an outbreak of food poisoning with incubation period of 2–4 hours after a hotel buffet. The epi-curve shows a single sharp peak. Attack rate analysis by food item shows: Rice (50% ate, AR 10%), Chicken curry (60% ate, AR 45%), Salad (40% ate, AR 12%), Dessert (70% ate, AR 11%). The vehicle MOST likely responsible is:
154. A researcher studying the effect of occupational pesticide exposure on Parkinson's disease controls for age, gender, and smoking in a multivariable logistic regression. After adjustment, the OR changes from 3.8 (crude) to 2.1 (adjusted). This change is BEST explained by:
155. In a prospective cohort study of lung cancer incidence, healthy worker effect is a concern. This bias leads to:
156. A study uses propensity score matching to control for confounding in a retrospective cohort. The propensity score is the:
157. An ecologic study examining the relationship between per-capita fat consumption and breast cancer rates across 20 countries shows a positive correlation. The main limitation of this study design is:
158. In an RCT comparing two drugs, despite randomisation, the drug-A group has more smokers at baseline. This imbalance is BEST addressed by:
159. Which of the following is the STRONGEST design for establishing causality between an exposure and an outcome?
160. A case-control study on bladder cancer and artificial sweetener use yields an OR of 1.8. After stratifying by gender, the OR in males is 1.7 and in females is 1.9, both close to the crude OR. This finding suggests:
161. A case-control study and a cohort study are designed to study the same risk factor. Which measure of association can be directly calculated from BOTH designs?
162. A prospective cohort study enrolls healthy individuals and finds that among 500 smokers followed for 10 years, 80 developed lung cancer, while among 500 non-smokers, 10 developed lung cancer. Calculate the attributable risk percent (attributable fraction) among exposed:
163. A researcher designs a study in which subjects are randomly assigned to either a new antihypertensive drug or placebo and followed for 5 years. At the end of the study, 60 of 300 drug-treated patients had cardiovascular events versus 90 of 300 controls. Calculate the Number Needed to Treat (NNT):
164. In a study on risk factors for hepatocellular carcinoma, only cases who survived long enough to be interviewed were enrolled. This introduces which type of bias?
165. A crossover trial compares two analgesics. After the first treatment period, a 2-week washout period is included before crossing over to the other treatment. The purpose of the washout period is primarily to:
166. An ecological study finds a strong positive correlation (r = 0.88) between per-capita sugar consumption and type 2 diabetes prevalence across 20 countries. A researcher concludes that sugar consumption causes diabetes in individuals. This fallacy is known as:
167. Which of the following study designs provides the strongest evidence for estimating the incidence of a rare disease with a long latency period and minimal bias?
168. A study compares mortality rates between two cities. City A has a crude death rate (CDR) of 18/1000, while City B has a CDR of 12/1000. After age-standardization using the direct method with a standard population, both cities show an age-standardized rate of 14/1000. The most likely explanation is:
169. A pharmaceutical company researcher reviews only published studies on a new drug and finds it significantly effective. However, several negative studies were never published. This introduces which type of bias?
170. In a randomized controlled trial on hypertension management, the blood pressure of participants in the control group improves significantly due to the awareness that they are being monitored. This phenomenon is called:
171. A cohort study finds that alcohol consumption is associated with liver cirrhosis (RR = 4.5). However, cigarette smoking, which is more common among heavy drinkers, is independently associated with liver cirrhosis. The apparent association between alcohol and cirrhosis is distorted by smoking. This is an example of:
172. In a systematic review and meta-analysis of RCTs evaluating aspirin for primary prevention of cardiovascular disease, heterogeneity among study results is assessed using which statistic?
173. A study evaluating a new cancer screening test shows that screened patients survive longer after diagnosis than unscreened patients, but overall mortality is the same in both groups. This spurious appearance of improved survival due to earlier diagnosis without change in death timing is called:
174. A researcher studying the relationship between dietary fat intake (exposure) and breast cancer (outcome) uses a cross-sectional design. The major limitation of this study design in establishing causality is:
175. In a case-control study of bladder cancer, 60 cases and 40 controls had occupational exposure to aromatic amines, while 30 cases and 70 controls were unexposed. The odds ratio is:
176. A prospective cohort study follows 200 exposed and 200 unexposed individuals for 10 years. Forty exposed and 20 unexposed develop the disease. The relative risk is:
177. Berkson's bias is a systematic error that occurs specifically in which type of epidemiological study?
178. A study reports a statistically significant association (p=0.03) between coffee consumption and pancreatic cancer. However, coffee drinkers also smoke more heavily, and smoking is a known risk factor for pancreatic cancer. The bias operating here is:
179. In systematic reviews and meta-analyses, the I² statistic is used to measure:
180. A surveillance system detects a sudden cluster of gastroenteritis cases in a village all falling within a 72-hour window, with an epidemic curve showing a steep rise and fall. This pattern is most consistent with:
181. A case-control study finds OR = 3.2 for pesticide exposure and non-Hodgkin lymphoma. Cases were recruited from tertiary cancer hospitals while controls were healthy hospital visitors. This is MOST likely an example of:
182. In a prospective cohort study, 500 non-smokers and 500 smokers are followed for 10 years. At the end, 20 non-smokers and 80 smokers develop COPD. What is the Population Attributable Risk Percent (PAR%) if the prevalence of smoking in the population is 30%?
183. In a systematic review of RCTs, the funnel plot shows asymmetry with studies clustered in the upper-right corner but few small studies with negative results. This pattern MOST suggests:
184. A Mendelian Randomisation study uses a genetic variant (SNP) as an instrumental variable to assess the causal effect of LDL cholesterol on coronary artery disease. Which assumption of this method is violated if the SNP also independently influences triglyceride levels?
185. A positive likelihood ratio (LR+) of 12 for a rapid antigen test means:
186. A prospective cohort study follows 5000 smokers and 5000 non-smokers for 10 years for lung cancer incidence. This study design best measures:
187. In a case-control study, cases of myocardial infarction are compared with age- and sex-matched controls for prior use of hormone replacement therapy. Exposure data is collected by interview. The most significant bias operating in this study is:
188. The Population Attributable Risk (PAR%) for a risk factor with a Relative Risk of 4 and a population exposure prevalence of 30% is approximately:
189. An ecological study finds that countries with higher fat consumption have higher rates of breast cancer. The error in concluding that high fat intake in individuals causes breast cancer based on this finding is called:
190. In a screening programme for cervical cancer, a test with high sensitivity is preferred. The primary reason for this is:
191. In an RCT comparing two drugs, the p-value is 0.03. Which of the following statements is TRUE?
192. In a cohort study evaluating the association between smoking and coronary artery disease, the crude relative risk is 3.0. After stratification by hypertension status, the RR in hypertensives is 3.1 and in non-hypertensives is 2.9. The Mantel-Haenszel pooled RR is 3.0. This finding is best described as:
193. A researcher wants to control for multiple confounders simultaneously in a case-control study examining oral contraceptive use and breast cancer. The most appropriate analytical technique is:
194. In a directed acyclic graph (DAG), a variable that is a common effect of both the exposure and the outcome is called a:
195. A systematic review comparing two antihypertensives calculates I² = 78%. The most appropriate next step in the meta-analysis is:
196. In a case-control study, 200 cases and 200 controls are enrolled. Exposure prevalence among cases is 60% and among controls is 40%. The odds ratio is:
197. Berkson's bias is a type of selection bias that occurs specifically in:
198. A study of 10,000 persons finds that the standardized mortality ratio (SMR) for a certain industry is 1.5 (95% CI: 1.2–1.9). The most appropriate interpretation is:
199. In a randomized controlled trial, allocation concealment refers to:
200. In an ecological study, the fallacy of inferring individual-level associations from group-level data is called:
201. In an intention-to-treat (ITT) analysis, participants are analyzed according to:
202. In a cohort study, 1000 smokers and 1000 non-smokers are followed for 10 years. Lung cancer develops in 50 smokers and 5 non-smokers. The attributable risk percent (ARP) in smokers is:
203. The Bradford Hill criterion 'consistency' in establishing causality means:
204. A cross-sectional study measures both exposure and outcome at the same point in time. The major limitation of this design for causal inference is:
205. In a prospective cohort study on diet and colorectal cancer, participants who join the study tend to be health-conscious, physically active, and have lower cancer risk than the general population. This causes:
206. A cohort study follows 500 smokers and 500 non-smokers for 10 years. During follow-up, 50 smokers and 10 non-smokers develop COPD. What is the Relative Risk (RR) of COPD in smokers compared to non-smokers?
207. In a case-control study investigating the association between occupational asbestos exposure and mesothelioma, investigators compare exposure history in 80 mesothelioma cases and 160 matched controls. The Odds Ratio (OR) is 12.4. Which type of bias is MOST likely to affect this study?
208. A study finds a higher incidence of hypertension in coffee drinkers. Further analysis reveals that coffee drinkers also smoke more than non-coffee drinkers, and smoking is an independent risk factor for hypertension. Smoking in this scenario is best described as:
209. The Attributable Risk Percent (AR%) in the exposed is 60% for a particular exposure-disease pair. What does this mean?
210. Bradford Hill's criteria are used to assess causal inference in epidemiology. Which criterion states that the exposure must precede the outcome in time?
211. A randomised controlled trial assigns eligible hypertensive patients to intervention or control by a random number table. Despite randomization, baseline characteristics reveal that the intervention group had significantly more diabetics. This is best explained by:
212. In epidemiology, a cross-sectional study measuring both exposure and disease simultaneously is most susceptible to which type of problem?
213. A researcher studying the association between dietary fat intake and coronary artery disease finds that physically active individuals tend to both eat less fat and have lower CAD rates. Physical activity is acting as a:
214. In a cross-sectional study on diabetes prevalence, diabetics with complications attend hospital more frequently and are overrepresented in the sample. This is an example of:
215. A Mendelian Randomization study uses genetic variants as instrumental variables to test the causal effect of LDL cholesterol on cardiovascular disease. Which property of the genetic variant is ESSENTIAL for it to be a valid instrumental variable?
216. In a study comparing two treatments for hypertension, the investigators did not know which treatment each patient received. This design element is called:
217. A funnel plot in a meta-analysis shows marked asymmetry with fewer small studies on the left side. The MOST likely explanation is:
218. Which study design provides the STRONGEST evidence for causality in the evidence hierarchy?
219. A disease surveillance system detects an outbreak of food poisoning in a district. Plotting the number of cases by date of symptom onset produces a curve that rises sharply and falls rapidly over 2 days. This pattern is called a:
220. Which of the following is the BEST measure of the strength of association between smoking and lung cancer in a case-control study?
221. A researcher conducts a case-control study on pesticide exposure and non-Hodgkin lymphoma. Cases are recruited from cancer hospital registries (which over-represent advanced cases), while controls are selected from the same hospital with other diagnoses. Which combination of biases is MOST likely operating?
222. In a prospective cohort study on alcohol and oesophageal cancer, 1200 exposed (heavy drinkers) and 3600 unexposed individuals are followed for 10 years. 48 exposed and 36 unexposed develop oesophageal cancer. The attributable risk percent (AR%) in the exposed is closest to:
223. A systematic review pools 8 trials on antihypertensive therapy. Funnel plot asymmetry is detected with small studies showing larger treatment effects. The I² statistic is 72%. Which combination of findings BEST describes the state of the evidence?
224. In a community survey, the odds ratio for developing dengue among households with overhead tanks versus piped supply is 3.2, and the prevalence of overhead tank use is 40%. The population attributable risk percent (PAR%) is approximately:
225. An investigator uses data from National Family Health Survey to examine the association between maternal education level (low/medium/high) and under-5 mortality, adjusting for wealth quintile and residence. This study design is BEST classified as:
226. Which of the following is the MOST appropriate measure to quantify the precision of an estimate in a clinical trial?
227. Herd immunity threshold (HIT) for a disease with basic reproduction number (R₀) of 5 is:
228. A clinical trial randomises 400 patients (200 per arm) to drug vs placebo. The primary outcome (all-cause mortality) occurs in 20% of drug arm vs 30% in placebo arm. What is the Relative Risk Reduction (RRR) and Number Needed to Treat (NNT)?
229. A researcher wants to study whether low birth weight is associated with adult-onset type 2 diabetes. The MOST efficient and ethical study design is:
230. In a meta-analysis on hypertension and stroke risk, the pooled odds ratio is 3.5 with 95% CI of 1.2 to 10.2. The BEST interpretation is:
231. Intention-to-treat (ITT) analysis in a randomised controlled trial is preferred over per-protocol analysis because ITT:
232. In an ecological study examining mean national salt consumption vs age-adjusted hypertension prevalence across 20 countries, a strong positive correlation (r=0.82) is observed. Which fallacy must be specifically cautioned against when interpreting this finding?
233. A researcher enrolls patients with a newly diagnosed disease and a matched group without the disease, then looks back to assess past exposure. This is best described as which study design?
234. Neyman bias (prevalence-incidence bias) occurs in which type of study?
235. In a meta-analysis, a funnel plot is used to detect:
236. Berkson's bias is characteristically associated with which study design?
237. A study finds that the association between coffee drinking and myocardial infarction disappears after stratifying by smoking status. This phenomenon is best described as:
238. Which measure of association is EXCLUSIVELY used in case-control studies as the primary measure?
239. A case-control study finds that 60 of 100 cases and 30 of 100 controls had prior statin use. What is the odds ratio for statin use in relation to the outcome?
240. In an RCT evaluating a new antihypertensive, allocation concealment is used. The PRIMARY purpose of allocation concealment is to prevent:
241. A cohort study of smokers shows a relative risk of 5.0 for lung cancer. The baseline incidence in non-smokers is 0.004 per year. What is the attributable risk percent (etiological fraction) in the exposed group?
242. A researcher studying diet and colorectal cancer selects controls from the same hospital wards as cases. This may introduce which type of bias?
243. In a cross-sectional study, the researcher finds a positive association between antihypertensive use and hypertension. This is MOST likely an example of:
244. In a matched case-control study, 10 pairs are concordant for exposure, 20 pairs have only the case exposed, and 8 pairs have only the control exposed. What is the matched odds ratio?
245. A community survey reports the prevalence of anaemia in adolescent girls as 56%. The researcher wants to estimate the prevalence with a 5% allowable error and 95% confidence. Which factor does NOT affect the required sample size in this calculation?
246. A cohort study reports a relative risk (RR) of 2.4 for myocardial infarction among hypertensives compared to normotensives. The population prevalence of hypertension is 30%. What is the Population Attributable Risk Percent (PAR%)?
247. Bradford Hill's criterion of 'biological gradient' (dose-response) is demonstrated in a study showing increasing lung cancer risk with increasing pack-years of smoking. Which of the following is MOST accurately described as a separate Bradford Hill criterion that strengthens causal inference?
248. In a case-control study of pesticide exposure and Parkinson's disease, cases are recruited from a neurological ward and controls from an orthopaedic ward. Cases with pesticide exposure are more likely to recall and report exposure than controls. This BEST exemplifies:
249. Effect modification (interaction) differs from confounding in that:
250. A systematic review uses Funnel plot asymmetry to assess potential bias. An asymmetric funnel plot with absence of studies in the lower left quadrant MOST suggests:
251. An investigator studies whether income is a confounder in the association between physical activity and type 2 diabetes. Which criterion must income satisfy to be classified as a confounder?
252. A cross-sectional study finds a prevalence odds ratio of 3.2 for obesity among adults with depression. The MAIN limitation of using prevalence (rather than incidence) data to study this association is:
253. The I² statistic in a meta-analysis is 75%. What does this BEST indicate?
254. A researcher investigates a new vaccine by randomly assigning 10 villages to vaccination and 10 to control, but treating all individuals within each village as one unit. This design is called a:
255. In a cohort study, the standardized mortality ratio (SMR) for a group of coal miners compared to the general population is 1.8 (95% CI: 1.4-2.3). This means:
256. In a cross-sectional survey, a significant association is found between high waist circumference and Type 2 diabetes. The MOST important limitation in inferring causality from this study is:
257. A Mendelian randomization study uses single nucleotide polymorphisms (SNPs) as instrumental variables for estimating causal effects of an exposure. This design is preferred over observational studies because SNPs:
258. An investigator studying the incidence of diabetes in a cohort notices that participants who are healthier and more compliant with follow-up have better outcomes compared to those who drop out. Over time, only the healthiest remain in the cohort. This is best described as:
259. A case-control study recruits cases from tertiary hospital and controls from the same hospital for a different condition. Cases are more likely to have hypertension as a 'risk factor' for the outcome. However, hypertension independently leads to hospitalisation. This is:
260. In a randomised controlled trial, the analysis includes all patients as randomised regardless of whether they completed the protocol or switched treatment arms. This is called:
261. A prospective cohort study follows 5000 non-hypertensive individuals aged 40–60 for 10 years. At the end of the study, 500 develop hypertension. The person-time at risk is 42,000 person-years (accounting for dropouts). The Incidence Density Rate (IDR) of hypertension is:
262. In a double-blind randomized controlled trial, 'blinding' refers to masking allocation from:
263. Ecological fallacy (Ecological bias) occurs when:
264. The 'attributable risk percent' (aetiologic fraction) in the exposed group is best defined as:
265. The GRADE (Grading of Recommendations Assessment, Development and Evaluation) system rates the quality of evidence from RCTs initially as 'High' which may be downgraded. Which factor can UPGRADE the quality of evidence from observational studies?
266. A propensity score matched study is conducted comparing outcomes of surgical versus medical management of stable CAD. What is the primary purpose of propensity score matching in this context?
267. In an RCT, the CONSORT statement requires reporting of which of the following to assess internal validity?
268. An occupational cohort study follows factory workers exposed to benzene for 15 years and compares cancer incidence with the general population. The Standardised Mortality Ratio (SMR) is found to be 0.72. This MOST likely reflects:
269. A matched case-control study for a rare disease (prevalence 0.5%) is designed with 1 case per 4 matched controls. This matching strategy is MOST appropriately analysed using:
270. In a clinical trial, 'intention-to-treat' (ITT) analysis means:
271. In a case-control study, the exposure odds ratio (OR) is 2.8 with 95% CI 1.6–4.9. When the disease is rare (prevalence < 5%), the OR BEST approximates:
272. A community health survey finds that a new ELISA test for leprosy has sensitivity 78% and specificity 92%. When used to screen a population with a disease prevalence of 2%, what phenomenon MOST challenges the utility of this test?
273. A researcher conducts a cross-sectional study and calculates a Prevalence Odds Ratio (POR) of 3.2 for obesity and hypertension. A colleague argues this overestimates the true association compared to a cohort study. Under which condition is the POR a reasonable approximation of the Incidence Rate Ratio (IRR)?
274. In a cluster randomised trial (CRT) evaluating a community nutrition intervention, the intracluster correlation coefficient (ICC) is 0.05 with cluster size of 50. What is the Design Effect (DEFF)?
275. The 'Bradford Hill criteria' are used to establish causal inference in observational epidemiology. Which criterion states that the exposure must precede the disease outcome?
276. A Directed Acyclic Graph (DAG) shows: Smoking → Lung Cancer, Smoking → Cardiovascular Disease, and Air Pollution → Lung Cancer. If one adjusts for Cardiovascular Disease in a study of Smoking → Lung Cancer, this introduces: