Understanding Relative Risk in Cohort Studies
In a cohort study comparing bad diet to good diet, relative risk measures the ratio of the probability (risk) of developing heart disease in the exposed group (bad diet) compared to the probability of developing heart disease in the unexposed group (good diet).
What Relative Risk Actually Measures
Relative risk (RR) is fundamentally a comparative measure that quantifies the probability of an outcome occurring in an exposed group relative to an unexposed group 1. In the context of your cohort study:
- RR compares the risk of heart disease between the two diet groups 2
- It is calculated as: RR = (Risk in exposed group) / (Risk in unexposed group) 1
- The "exposed" group would be those with the bad diet, and the "unexposed" reference group would be those with the good diet
Why the Answer is Option B (with clarification)
The most accurate answer is Option B: "Risk of heart disease in exposed group" - though this requires important clarification. The relative risk doesn't measure the absolute risk in the exposed group alone, but rather the ratio of risk in the exposed group compared to the unexposed group 1.
Breaking Down Each Option:
Option A (Odds): This confuses relative risk with odds ratio. While odds ratios are used in case-control studies, relative risk directly measures probability ratios in cohort studies 3, 1
Option B (Risk in exposed group): This is the closest answer, as RR fundamentally relates to the risk in the exposed group, though technically it's a comparison of that risk to the unexposed group 1
Option C (Probability based on diet): This is too vague and doesn't capture the comparative nature of relative risk 1
Key Distinctions in Cohort Studies
In cohort studies specifically, relative risk can be directly calculated because you follow both exposed and unexposed groups forward in time 3, 1. This differs from case-control studies where:
- You cannot directly calculate relative risk 3
- Odds ratios are used instead 3
- The odds ratio approximates the risk ratio only when the outcome is rare (<10% incidence) 3, 4
Clinical Context
When interpreting relative risk from dietary cohort studies examining cardiovascular outcomes:
- RR > 1 indicates increased risk in the exposed (bad diet) group 1
- RR < 1 indicates decreased risk (protective effect) in the exposed group 1
- RR = 1 indicates no difference in risk between groups 1
The relative risk provides a measure of association strength between dietary exposure and cardiac disease outcomes in population-based research 2.