Number of practices required for cluster randomised trial (CRT) of interventions for typical mother and child health outcomes, assuming 50 cases/practice
| Measure | Effect size |
Number for individual RCT |
Inflation factor (IF) |
Number of practices for CRT |
| EPDS (Q1) | 8.5 → 7.5 | 882 | 2.42 | 43 |
| EPDS ⩾ 13 (Q1) | 25% → 15% | 540 | 2.65 | 29 |
| EPDS (Q2) | 7.5 → 6.5 | 754 | 2.17 | 33 |
| EPDS ⩾ 13 (Q2) | 15% →10% | 1450 | 1.53 | 44 |
| Mother smokes (Q1) | 28% → 23% | 2462 | 2.31 | 114 |
| Mother smokes (Q2) | 25% → 20% | 2266 | 5.31 | 241 |
| Breast feeding under 6 months | 35% → 45% | 790 | 2.74 | 43 |
| Breast feeding at birth | 75% → 85% | 540 | 1.67 | 18 |
| Sleep problem (Q1) | 12% → 8% | 1862 | 1 | 38 |
| Sleep problem (Q2) | 12% → 8% | 1862 | 1 | 38 |
| General health (WCHMP) | 50% → 60% | 814 | 2.41 | 39 |
| Minor illness (WCHMP) | 8.0% → 6.5% | 9646 | 3.15 | 607 |
| >2 minor illness (WCHMP) | 19% → 15% | 2866 | 3.35 | 192 |
| Accidents (WCHMP) | 9% → 7.5% | 10824 | 1 | 216 |
| Behaviour (WCHMP) | 6.5% → 4.5% | 3924 | 2.01 | 158 |
| Hospitalised in past 6 months (WCHMP) | 9% → 7.5% | 10824 | 1 | 216 |
| Hospitalised in past 2 months (Q1) | 15% → 12% | 4204 | 1.69 | 142 |
| Seen GP in past 2 weeks (Q1) | 33% → 25% | 1058 | 1 | 21 |
| Seen GP in past 2 weeks (Q2) | 23% →18% | 2124 | 1 | 42 |
| GP visits over 1 year (mother) | 3.2 → 2.5 | 576 | 2.36 | 27 |
| GP visits over 1 year (child) | 4.2 → 3.5 | 785 | 2.34 | 37 |
These values assume 80% power to detect the stated difference with a two tailed significance of 95%.
EPDS, Edinburgh postnatal depression score; GP, general practitioner; Q, questionnaire; RCT, randomised controlled trial; WCHMP, Warwick child health and morbidity profile.