Sample size
We had no prior data from SSA. We assumed that the mean (SD) glucose level 2 h following a 75 g oral glucose load would be 10.0 (1.5) mmol/l at 12 months in the placebo arm. We calculated that a study with 160 people per arm would have 85% power to detect a difference in blood glucose levels at 2 h post a 75 g glucose load from 10 mmol/l down to 9.5 mmol/l in the metformin arm (i.e. an absolute difference of 0.5 mmol/l between the two arms) and enroled 182 people per arm to allow for losses to follow-up.