Retrospective studies
Warburton and Fraser (1964)
Stevenson et al. (1959)
Leridon (1976)
Naylor and Warburton (1978)

12.3

15.2
11.0

26.2
16.8
22.0
20.3

32.2
19.2
35.3
29.2

30.2
26.2

37.0

Cohort studies
Shapiro et al. (1970)
Awan (1974)
Prospective studies
Boué et al. (1975)
Lauritsen (1976)
Harger et al. (1983)
Fitzimmons et al. (1983)

10.9
10.4

18.0
22.1


13.8
13.2

27.4


17.4
31.3

24.9
32.5
29.2
45.7