What does relative risk (RR) measure in a cohort study examining the effect of diet on cardiac disease?

What Relative Risk Measures in a Cohort Study

Relative risk (RR) measures the ratio of the probability of an outcome occurring in the exposed group compared to the unexposed group—it directly compares the risk of disease between those with the exposure (bad diet) versus those without it (good diet).

Understanding Relative Risk

Relative risk is the fundamental measure of association in cohort studies that quantifies how many times more (or less) likely the exposed group is to develop the outcome compared to the unexposed group 1.

Key Characteristics of RR:

• RR directly calculates the risk ratio by dividing the incidence rate in the exposed group by the incidence rate in the unexposed group 1
• An RR of 1.0 indicates no association between exposure and outcome 2
• RR > 1.0 indicates increased risk in the exposed group (e.g., RR = 1.97 means 97% higher risk) 2
• RR < 1.0 indicates decreased risk in the exposed group (e.g., RR = 0.80 means 20% lower risk) 3

Application to Your Diet-Cardiac Disease Study

In the cohort study examining diet and cardiac disease:

• The RR would compare the incidence of cardiac disease in the "bad diet" group to the incidence in the "good diet" group 2
• For example, if RR = 1.45, this means the bad diet group has 45% higher risk of cardiac disease compared to the good diet group 2
• Conversely, a healthy dietary pattern typically shows RR = 0.80, meaning 20% lower cardiac disease risk 3

Why Not the Other Options

Option A (Odds of heart disease in exposed group):

• This describes an odds ratio (OR), not relative risk 1
• OR is typically used in case-control studies, not cohort studies 1
• When outcomes are common (>10%), OR overestimates the true RR 1

Option B (Risk of heart disease in exposed group):

• This describes absolute risk in one group only, not a comparison measure 1
• RR requires comparing two groups, not describing a single group 1

Option C (Probability based on diet):

• This is too vague and doesn't capture the comparative nature of RR 1
• RR specifically quantifies the ratio between groups, not just probability 2

Clinical Context from Diet Studies

Meta-analyses of dietary patterns and cardiac disease consistently report results as relative risks 3:

• Prudent/healthy dietary patterns: RR = 0.80 (20% risk reduction) 3
• Western/unhealthy patterns in US populations: RR = 1.45 (45% risk increase) 3
• These RR values directly compare disease incidence between dietary exposure groups 2