Sample size calculation

Suppose that we are prepared to accept that this is likely to be confirmed in the third batch, i.e. that we can combine the results for C and B throughout the 3 batches for a comparison with A+B. The formula for t now becomes

where and now correspond to diets C and B pooled and diet A+B, respectively.

Suppose that n₁ = 2n₂ and let us write n₂ as n. Using this new formula it can be shown that

n = (3/2 t² s²) / ( )²

Substituting values from the GenStat output, as before, we obtain

n = (1.5 x 2² x 1653) / (219.9 - 187.25)²

where t = 2 and 187.25 = average of 188.4 for diet C and 185.7 for diet B. Thus, n = 9.3 or, rounding up to the next integer, n =10.