Relative Risk
Definition: Relative risk measures the magnitude of an association between an exposed and non-exposed group. It describes the likelihood of developing disease in an exposed group compared to a non-exposed group.
Relative risk is calculated using cumulative incidence data to measure the probability of developing disease. Therefore, relative risk can only be calculated if the study was designed in such a way that incidence data can be calculated. For example, study designs such as cohort studies and clinical trials allow the researcher to calculate incidence, whereas case-control studies do not. Thus, relative risk can be calculated for cohort studies and clinical trials, but not for case-control studies. Odds ratios can be used to estimate relative risk for a case-control study.
Relative risk is:
(Cumulative incidence in the exposed)/(Cumulative incidence in the unexposed)
Calculating Relative Risk
Relative risk can be calculated from a 2x2 table using the following formula: Relative Risk = (A / (A+B)) / (C / (C+D)).
| Disease Yes | Disease No | Total | |
| Exposure Yes | A | B | A+B |
| Exposure No | C | D | C+D |
| Total | A+C | B+D | A+B+C+D |
Example: In a study of 1000 people, investigators observed who was bitten by a flea and who subsequently developed plague.
From the data from 1000 people, here is the 2x2 table and relative risk calculation.| Plague Yes |
Plague No |
Total | |
| Flea Bite Yes |
400 | 200 | 600 |
| Flea Bite No |
100 | 300 | 400 |
| Total |
500 | 500 | 1000 |
Relative Risk = (A / (A+B) ) / (C / (C+D)) =
(400 / (400+200)) / (100 / (100+300)) = (400 / 600) / (100 / 400)
= (0.667/0.25) = 2.67
Interpretation: People with flea bites had a 2.7 times greater risk of developing plague than those without flea bites.
