Question

A researcher plots the average per-capita alcohol intake of 10 different countries against the cirrhosis mortality rate of each of those countries, using one data point per country, to look for an association. Which type of epidemiological study design does this represent?

• A. Ecological study
• B. Cohort study
• C. Case-control study
• D. Cross-sectional study

Hint:

The unit of analysis is the whole country (a group), not the individual.

Answer 1

The Correct Option is A

Step 1: Identify the unit of observation. In this study each data point is an entire country (a population/group), not an individual person. Per-capita alcohol intake and cirrhosis mortality are both group-level (aggregate) measures averaged over the whole country.

Step 2: Apply the definition of an ecological study. An ecological (correlational) study examines the relationship between an exposure and an outcome at the level of populations or groups rather than individuals. Comparing the mean exposure of a group with the mean outcome of that same group across several groups is the hallmark of this design. Here, 10 countries serve as the 10 groups.

Step 3: Why the others are wrong. A cohort study follows exposed and unexposed individuals over time for incidence — not the case here. A case-control study starts from individuals with and without disease and looks back at exposure — again individual-level. A cross-sectional study measures exposure and outcome in individuals at one point in time. All three use the individual as the unit of analysis, whereas this study uses the country.

Step 4: Note the limitation. Because data are aggregated, an ecological study is prone to the ecological fallacy — a group-level association may not hold for individuals. It is best used for hypothesis generation.

Key fact: Comparing average alcohol intake versus cirrhosis across 10 countries (group as the unit) is an ecological study.