In a community survey of 500 adults, the mean systolic BP is 126 mmHg with a standard deviation of 14 mmHg. Assuming a normal distribution, what percentage of individuals are expected to have systolic BP between 112 and 140 mmHg?
- A 68.3%
- B 99.7%
- C 95.4% ✓
- D 90.0%
Correct answer: C. 95.4%
Explanation
Mean ± 1 SD = 126 ± 14 = [112, 140]. In a normal distribution, mean ± 1 SD encompasses 68.3%, mean ± 2 SD encompasses 95.4%, and mean ± 3 SD encompasses 99.7% of values. Since 112 and 140 each lie exactly 1 SD from the mean, 68.3% is the answer for ±1 SD. However, 112 = 126 − 14 (1 SD below) and 140 = 126 + 14 (1 SD above) — this is precisely mean ± 1 SD = 68.3%.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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