How to calculate the degree of freedom to the dependent t-test?

Answer 1

You should think of a dependent t as being a single-sample t on the difference scores. This gives it 1 less than the number of differences as the df.

Say you have before/after scores for 10 people. You have 20 scores, but the test is done on the differences, of which you have 10 and that means 9 df. You typically obtain df from n - 1, as you do in this case, you just need to be careful to think of this as the number of pairs and not scores.