Relative Risk

We often use Relative Risk (RR) to compare the likelihood or chance of an event occurring between two groups. Relative Risk is a descriptive statistic, not an inferential one, 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).  We calculate Relative Risk by dividing the probability of an event occurring for group 1 (A) by the probability of the event occurring for group 2 (B). 

Relative Risk is similar to Odds Ratio; however, we calculate RR using percentages, while we calculate Odds Ratio 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).