Epidemiology – Relative Risk (RR)

Firstly, a few points need to be made regarding what is meant by risk

  • Risk = the statistical likelihood of having an adverse event (e.g. illness or death) following exposure to some factor
  • Risk is a measure of association NOT causation… it cannot tell us about the likelihood of harm


Relative Risks (RR) are used to compare the risks of different groups.   Defined as:

The probability that a member of an exposed group will develop disease relative to the probability that a member of an unexposed group will develop the same disease

  • As such RR measure the strength of association between an the risk of exposure and an outcome, compared to the risk of non-exposure and the same outcome.
  • RR can be assessed using 3 calculations: risk ratio, rate ratio and odds ratio
  • Risk calculations require all knowledge of those exposure and unexpose
  • It is the risk of developing the disease (or outcome) relative to the exposure

Risk Ratio

  • Risk Ratio = (risk of disease in the exposed) / (risk of disease in the non-exposed)
  • Requires complete follow-up of data – calculation of risk is based on the population at risk at the start of the study
  • Risk Ratio doesn’t account for time to event between groups, only final outcome
  • Risk ratio is most appropriate to assess protective effects of an intervention (e.g. vaccinations)

Rate Ratio

  • Rate Ratio = (rate of disease in the exposed) / (rate of disease in the non-exposed)
  • Calculation of rate is based on the total person-years at risk during the study, therefore reflecting the changing poplation at risk
  • Preferential choice for longitudinal studies as it incorporates changes over time

Odds Ratio

  • Odds Ratio = (odds of disease in exposed) / (odds of disease in the non-exposed)
  • Always the measure of association for case-control studies
  • For rare diseases (or diseases with long latency periods) the OR can be an approximate measure to the RR (relative risk)
  • Doesn’t require denominator (i.e. total number in population) unlike measuring risk
  • For more info, visit my blog post on Odds Ratios here


  • RR is a measure of association, as such as cannot infer causation from any of these calculations
  • RR assesses the risk of developing disease relative to exposure… *but* gives no indication of the magnitude of the excess risk in absolute terms.  For this we need to understand the Attributable Risk
  • Can sometimes be confusing deciding which RR calculation to use when… best advice is to think about a) what is the study design? (OR is always used for case-control) and b) has follow-up data been completed for all participants? (Yes = risk ratio, No = rate ratio)

 NB: I couldn’t find any ‘risk’ appropriate pictures, so here are some dogs having fun instead.  Enjoy!


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