Assuming a mean weight of 22.4 kg (SD=3.0 kg); and SFP is expected to increase the mean weight from 22.4 kg to 24.0 kg, thus to detect a difference of 1.6 kg with 90% power and type I error=0.05, design effect=2.0 and 10% drop-outs, the necessary sample-size calculated to be at least 164 school children in each group. This is less than the sample size required for BMI.