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Which of the following best describes a key flaw of cohort studies?

A Small sample sizes, which limit the statistical power of the analysis
B The use of retrospective records, which introduces recall bias at the outcome stage
C Loss to follow-up, which biases the comparison when dropouts differ from those who remain ✓
D The absence of a comparison group, which limits the interpretation of exposure effects
E Difficulty calculating incidence, because outcome events are not recorded during follow-up

The answer is C: Loss to follow-up, which biases the comparison when dropouts differ from those who remain

Explanation

Why C is correct

See Flaws of Cohort Studies. Option C is correct. In a cohort study, groups of people are classified by exposure and then followed to see who develops the disease. Loss to follow-up happens when participants die, refuse to continue or cannot be contacted, or when records are incomplete in a retrospective cohort. Fewer observed outcomes reduce precision, and the loss can also bias the comparison if the reasons for leaving are related to exposure, disease risk or both. For example, people who develop early symptoms of heart disease, such as angina, may be less likely to keep attending, so heart attacks are under-counted among those exposed. Restricting the analysis to those who remain can leave a healthier group (survivor bias). Researchers should record why people were lost and check how the loss relates to exposure and outcome. Cohort studies are also usually expensive and time-consuming because they need large samples and years of follow-up. A useful memory aid is that cohort studies lose people over time, and the people lost may be the ones that matter.

Cohort study: groups are classified by exposure and then followed to see who develops the disease.
Loss to follow-up: participants die, refuse to continue or cannot be contacted, which reduces precision.
Bias: the comparison is distorted if the reasons for leaving are related to exposure, disease risk or both.

Why the other options are incorrect

A. Small sample sizes, which limit the statistical power of the analysis: Cohort studies typically recruit large samples precisely to allow for losses to follow-up and adequate statistical power, so small sample sizes are not an inherent key flaw.
B. The use of retrospective records, which introduces recall bias at the outcome stage: Retrospective cohort studies use existing records to classify exposure and outcome, not participant recall, so recall bias is not introduced by design.
D. The absence of a comparison group, which limits the interpretation of exposure effects: A cohort study is defined by comparing groups with different exposure, so a comparison group is built in; interpretation is limited by confounding and bias, not by its absence.
E. Difficulty calculating incidence, because outcome events are not recorded during follow-up: Cohort studies follow participants over time and record outcome events during follow-up, so incidence can be calculated; this is a strength, not a flaw.

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1
In a case-control study comparing people with a disease to people without it, which measure is typically calculated to assess the association between a past exposure and the outcome?
2
A prospective cohort study follows two groups for 5 years to investigate the association between a chemical exposure and the development of a skin condition. In the exposed group, 10 cases occur over 500 person-years at risk. In the unexposed group, 5 cases occur over 500 person-years at risk. Which statement best describes the incidence rate ratio for the exposed group compared with the unexposed group?

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