13 Research methods, statistics, critical review and evidence-based practice13.2 Epidemiological studies
13.2.2.5 Appraising ecological studies
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Written by Resident Doctor Committee · Published
New in the 2027 syllabus
The College added this topic for exams from January 2027.
What makes a study ecological
An ecological study compares groups, such as regions, not people. It uses aggregate measures of exposure and outcome for each group, rather than linking each person's exposure to that person's outcome.
The ecological fallacy is assuming that an association between groups also holds for the individuals within them.
Individual correlational studies use paired observations from each person. A correlation coefficient alone does not make a study ecological.
Appraisal questions
Issue | What to check |
|---|---|
Comparable groups | Definitions, age structure and inclusion criteria match |
Measurement | Data sources and recording practices are comparable |
Timing | Exposure comes before the outcome, if a cause is claimed |
Confounding | Other group characteristics could explain the association |
Dependence | Neighbouring areas or repeated yearly measurements may be correlated |
For example, a link between regional prescribing and suicide rates cannot show whether the people prescribed treatment were the people who died.
Age standardisation reduces one source of difference between areas. It does not remove all confounding, at group or individual level.
Aggregation choices can change the association when boundaries or time periods change, so the grouping should be justified.
Drawing conclusions
Routine aggregate data are a cheap way to examine population patterns and generate hypotheses, especially when individual exposure data are unavailable.
Some questions are genuinely about populations. Aggregate analysis is then appropriate, but the conclusions should stay at the population level.
A significant correlation is not enough to claim cause. Reverse causation, common causes and differential ascertainment (outcomes being detected or recorded differently between groups) all remain possible.