# Relative Risk

Posted July 17, 2013

Relative Risk (RR) is often used when the study involves comparing the likelihood, or chance, of an event occurring between two groups.  Relative Risk is considered a descriptive statistic, not an inferential statistic; as it does not determine statistical significance.  Relative Risk utilizes the probability of an event occurring in one group compared to the probability of an event occurring in the other group.  It requires the examination of two dichotomous variables, where one variable measures the event (occurred vs. not occurred) and the other variable measures the groups (group 1 vs. group 2).  Relative Risk is calculated by dividing the probability of an event occurring for group 1 (A) divided by the probability of an event occurring for group 2 (B).  Relative Risk is very similar to Odds Ratio, however, RR is calculated by using percentages, whereas Odds Ratio is calculated by using the ratio of odds.  Relative Risk values are greater than or equal to zero.  A value of 1 indicates a neutral result: the chance of an event occurring for one group is the same for an event occurring for the other group.  However, a value of zero indicates that none of the cases in group 1 had the event occur while x number of cases in group 2 had the event occur; or in other words, the numerator was a zero (A = 0) and the denominator was any number greater than zero (B = x, where x > 0).

An example:

Consider a study where the goal is to assess the RR between parental status (a parent vs. not a parent) and intelligence level (low intelligence vs. high intelligence).  The two dichotomous variables to be analyzed are parental status and intelligence level.  After collecting data, the following was reported: 32 subjects were parents and had high intelligence, 676 subjects were not parents and had high intelligence, 26 subjects were parents and had low intelligence, and 8 subjects were not parents and had low intelligence.  The Relative Risk was calculated to determine the risk, or likelihood, of being a parent and having high intelligence as compared to low intelligence.  The calculated RR was reported to be 0.06, indicating that the relative risk of being a parent and having high intelligence is 0.06 times that of people who are parents and have low intelligence.