Table 1

Sample size calculations

Exposure comparison (ratio of unexposed: exposed)

Outcome of interest

Difference

Sample size estimate

Total number required*

Spontaneous vaginal birth:forceps/vacuum extraction (2:1)

Urinary incontinence

Faecal incontinence

Sexual problems

Perineal pain

11% versus 18%

2.6% versus 8%

24% versus39%

20% versus 53%

930

642

360

168

1200

800

450

210

Emergency caesarean:forceps/vacuum extraction (1:2)

Urinary incontinence

Faecal incontinence

Sexual problems

7% versus 18%

3.9% versus 10.5%

29% versus 39%

371

573

841

950

1500

2200

Elective caesarean:forceps/vacuum extraction (1:5)

Urinary incontinence

Faecal incontinence

Sexual problems

2.4% versus 18%

2.8% versus 10.5%

27% versus 39%

276

763

938

900

2500

3100

* Taking into account the relative frequency of different methods of delivery

Brown et al. BMC Pregnancy and Childbirth 2006 6:12   doi:10.1186/1471-2393-6-12