Table 1

Two studies (1 and 2) examining the association between choice of playing field and risk of hamstring injury among soccer players (fictive example)

Hamstring injury (n)No hamstring injury (n)Total (n)CIP (95% CI)RR (95% CI)RD (95% CI)
Study 1Turf200800100020.0% (17.5% to 22.5%)2.00 (1.60 to 2.50)10% (6.90 to 13.10)
Grass100900100010.0% (8.1% to 11.9%)1 (ref)0 (ref)
Study 2Turf2098010002.0% (1.13% to 2.87%)2.00 (0.94 to 4.25)1% (−0.06 to 2.06)
Grass1099010001.0% (0.38% to 1.62%)1 (ref)0 (ref)
  • The RRs are similar across the two studies, whereas the RDs differ. An RR of 2 is equivalent to a relative risk reduction of 100% in the grass group in both studies.

  • CIP, cumulative incidence proportion; Grass, natural grass; RD, risk difference (CIP turf minus CIP grass); Ref, reference; RR, relative risk (CIP turf/CIP grass); Turf, artificial turf.