Systematic sampling is used to select a study sample. Every 10th person from a numbered list of 500 is selected after a random start. What is the major risk specific to this method?
- A Non-response bias
- B Clustering of subjects from the same geographic area
- C Periodicity bias if the sampling interval coincides with a cyclical pattern in the list ✓
- D Inability to calculate sampling error
Correct answer: C. Periodicity bias if the sampling interval coincides with a cyclical pattern in the list
Explanation
The chief vulnerability of systematic sampling is periodicity (periodic bias) — if the list has a hidden cyclic pattern that coincides with the sampling interval, the sample will systematically over- or under-represent certain characteristics. For example, if a list alternates male-female-male-female and the sampling interval is 2, only males or only females will be selected. This is an inherent design risk distinct from non-response (a field execution problem) or clustering (a feature of cluster sampling).
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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