Repeated measures T test

Question

In a repeated measures T test why is the population mean 0?

Answer 1

In a repeated measures t test we are testing the claim that there is a significant difference between the two measurements (that the mean difference is significantly different from zero). Thus the null hypothesis is that the mean difference is zero, while the alternate hypothesis is that the mean difference is not zero (two-sided) or greater than or less than zero (one sided).