Sample size calculations for a definite RCT with primary outcome of BMI SDs at 24 months under different assumptions
| 80% power | 90% power | |||||
| Minimum difference detectable | Minimum difference detectable | |||||
| ICC | 0.25 | 0.30 | 0.50 | 0.25 | 0.30 | 0.50 |
| 0.01 | 571 | 397 | 145 | 764 | 531 | 193 |
| 0.03 | 947 | 658 | 240 | 1267 | 880 | 320 |
| 0.06 | 1511 | 1050 | 383 | 2021 | 1404 | 511 |
| 0.10 | 2262 | 1572 | 573 | 3027 | 2103 | 765 |
All numbers are the total number of children required to be recruited, assuming 50 children per school and a loss to follow-up at 24 months of 20%.
The calculations take into account the reduction in variability associated with adjusting for baseline BMI SDs, conservatively assuming the correlation between baseline and 24 months data to be 0.8 (in our pilot study, this correlation was 0.93).
This is the sample size required to detect a true minimal difference in BMI SDs of 0.25, 0.3 or 0.5, assuming the SD is 1.3 (based on our pilot data).
BMI, body mass index; ICC, intraclass correlation coefficient; RCT, randomised controlled trial.