Example showing how a continuous variable (age) can be transformed to a grouping variable to compare risk of injury (hamstring strains) between groups. The players have been split into three groups using the mean −1 SD (20 yrs) and the mean +1 SD (28 yrs) as the cutoff values: Younger, older, and an intermediate reference group. The data have been taken from Arnason et al44
| Younger group (age <20 yrs) | Reference group (age 20–28 yrs) | Older group (age >28 yrs) | |
|---|---|---|---|
| No. of players | 42 | 206 | 88 |
| No. of injuries | 0 | 11 | 8 |
| Injury risk (%) | 0 | 5.3 | 9.1 |