<Li> T is the treatment component, corresponding to the number of treatment groups (including control group) being used, or the number of questions being asked (minus 1) </Li> <Li> E is the degrees of freedom of the error component, and should be somewhere between 10 and 20 . </Li> <P> For example, if a study using laboratory animals is planned with four treatment groups (T = 3), with eight animals per group, making 32 animals total (N = 31), without any further stratification (B = 0), then E would equal 28, which is above the cutoff of 20, indicating that sample size may be a bit too large, and six animals per group might be more appropriate . </P> <P> Let X, i = 1, 2,..., n be independent observations taken from a normal distribution with unknown mean μ and known variance σ . Let us consider two hypotheses, a null hypothesis: </P>

How large a sample is necessary for this sample mean to be statistically significant