Risk Ratio versus Rate Ratio

Lead Author(s): Jeff Martin, MD


Relative risk or RR is very common in the literature, but may represent: There can be substantial difference in the association of a risk factor with In the general medical literature, rate is often incorrectly used for prevalence measures.

These inconsistencies are even greater and are compounded by the fact that an abbreviation for both a risk ratio and a rate ratio is RR.

Relative Risk

Risk is based on proportion of persons with disease = cumulative incidence Since all of the measures are ratios, either of probabilities or of odds, it is clearer and simpler to use the word ratio in describing each type.

Risk reflects the proportion of persons experiencing the event, so it follows that comparing two cumulative incidences is called a risk ratio.

Relative Rate

Rate is based on events per person-time = incidence rate Rate should be limited to measures of incidence based on person-time rates, so a ratio of two such measures is called a rate ratio.

Comparison: Risk Ratio and Rate Ratio

Risk must be a proportion; therefore, it must be between 0 and 1 Rates are not restricted between 0 and 1

Rate Ratio vs Risk Ratio - What do you report?

Is the risk ratio or 1.38 telling you something different than the rate ratio of 2.0?

Risk Ratio: Long-Term Probability of the Disease

You might prefer the risk ratio if your emphasis is on how different the long-term probability of the disease is for an exposed and an unexposed population.

Rate Ratio: Disease Causality

If your emphasis is on whether the exposure is causal for the disease, the rate ratio would be preferable because it preserves the larger relative force on disease occurrence of being exposed compared to being unexposed.

So this comparison illustrates why we refer to the rate as the more fundamental measure of disease occurrence. When we form ratio (relative) measures with it, it is not subject to the artifact of the ceiling effect that we see here with ratio measures of two proportions.