Sample size calculations II

A randomised controlled trial investigated whether a computerised feedback device modified eating behaviour and resulted in weight loss in obese adolescents.1 The primary outcome was change in body mass index (BMI) from recruitment to 12 months. BMI was calculated as weight (kg)/[height (m)]², adjusted for age and sex. The control intervention was standard lifestyle modification therapy.

The optimal sample size to compare the computerised device with standard care was calculated for the primary outcome. Using data from a previous study, the researchers predicted that the mean change in BMI at 12 months with standard therapy would be a reduction of 0.17 (SD=0.267) kg/m². For the computerised device to be considered effective, it should double this mean reduction and achieve a decrease in BMI of 0.34 kg/m² (the smallest effect of clinical interest). A total sample size of 80 children …