Difference between two training programmes - which test? Mauchly's significance?


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I want to know if aerobic training is better than anaerobic training.
There are 20 people in each group who performed a timed test before and after training.
Can I do a t-test on the difference between the before and after scores per individual or do I need to do a two-factor mixed factorial ANOVA??

If you run a repeated measures ANOVA in SPSS and get no number showing in the significance box on a Mauchlys test of sphericity is it because there are only two within subjects variables or some other reason??