What Relative Risk Measures in a Cohort Study
Relative risk (RR) measures the ratio of the probability of disease occurring in the exposed group compared to the unexposed group—it directly compares the risk of heart disease between those with a bad diet versus those with a good diet. 1
Definition and Calculation
Relative risk is calculated as:
- RR = (Risk in exposed group) / (Risk in unexposed group) 1
- This represents how many times more (or less) likely the exposed group is to develop the outcome compared to the unexposed group 2
- An RR of 2.0 means the exposed group has twice the risk of disease compared to the unexposed group 3
What RR Actually Tells You
- RR quantifies the strength of association between an exposure (diet) and outcome (cardiac disease) 3
- It provides a measure of change in the likelihood of an event linked to a given exposure 4
- Values greater than 1.0 indicate increased risk in the exposed group; values less than 1.0 indicate decreased risk 3
Important Distinction from Other Measures
- RR is NOT the same as odds ratio (OR), though they are often confused 5
- RR directly measures risk (probability), while OR measures odds (ratio of affected to unaffected) 1
- In cohort studies like this one, RR can be directly calculated because you follow both groups forward in time 6
- When outcomes are common (>10%), OR will overestimate RR 5
Upper Limits and Interpretation
- The maximum possible RR is limited by the baseline incidence: it equals 1 divided by the baseline incidence rate 2
- For example, if baseline cardiac disease incidence is 10%, the maximum possible RR is 10 2
- Most diet-disease associations show modest RR values below 2.0 3
Answer to Your Question
Based on the options provided, the answer is C: Probability of having heart disease based on diet—because RR compares the probability (risk) of disease between the two diet groups 1. It is not simply the odds (Option A) or risk in only the exposed group (Option B), but rather the comparative ratio between both groups.