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