Can you use composite z-scores to perform a t-test. If so, can you use a composite z-scores in a repeated measures t-test?
Yes, you can use composite z-scores when doing a t-test. If you are doing repeated measures t-test it is essential that you use the same mean and standard deviation across time points to calculate your composite z-scores. Otherwise, any differences over time will cancel out, and the mean of both time points will be zero.
For more information about composite z-scores, see my post on Calculating Composite Scores of Ability and Other Tests . For some reason it has attracted 20 or 30 questions on the topic from various readers.