The effect size (d) comparing a mean yearly change of 120 g in the placebo group versus a change of 195 g in the Vitamin D3 and calcium group is therefore 0.89. Twenty-one children in each group will be sufficient to detect an effect size of this magnitude with alpha = 0.05 and beta = 80%. If the sample size is increased to 30 children in each group, power will be sufficient to detect a difference of 61 gm between the vitamin D3 and calcium group and the placebo group with alpha = 0.05 and power = 80%. This is comparable to the difference observed in a clinical trial of healthy adolescent girls (mean age, 11.9 years) who received either calcium (110% of RDA) or placebo (Lloyd 1996).