Relative Risk Calculator

What is Relative Risk?

Relative risk (RR), also called the risk ratio, compares the probability of an event in two groups. RR = P(event | exposed) / P(event | unexposed). It is the most intuitive measure of association for cohort studies and clinical trials because it directly answers: "How many times more likely is the event in the exposed group?"

Relative Risk Formula

For a 2×2 table:

Interpreting Relative Risk

Interpretation guidelines:

• RR = 1: No difference in risk between groups
• RR > 1: Increased risk in the exposed group (risk factor)
• RR < 1: Decreased risk in the exposed group (protective factor)
• 95% CI includes 1: The association is not statistically significant

Relative Risk vs. Odds Ratio

RR is preferred for prospective studies (cohort, RCT) because it directly represents probability ratios. The odds ratio is used in case-control studies where you cannot estimate incidence. For rare events (< 10%), OR ≈ RR. CrossTabs.com calculates both automatically for any 2×2 table.

Frequently Asked Questions

When should I use relative risk vs. odds ratio?

Use RR for cohort studies and clinical trials where you can calculate actual incidence rates. Use OR for case-control studies where the sampling is based on outcome status. For rare outcomes, they approximate each other.

What is the number needed to treat (NNT)?

NNT = 1 / absolute risk reduction = 1 / |P(event|exposed) − P(event|unexposed)|. It tells you how many patients need to be treated to prevent one additional adverse outcome. CrossTabs.com calculates NNT automatically.

Can relative risk be used for retrospective studies?

No. In case-control studies, the proportion of exposed vs. unexposed is determined by the researcher, so incidence rates cannot be estimated. Use the odds ratio instead.