Table 2

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 players4220688
No. of injuries0118
Injury risk (%)05.39.1