To construct an interval estimate for the difference between the means of two populations when the standard deviations of the two populations are unknown, we must use a t distribution with (let n1 be the size of sample 1 and n2 the size of sample 2) (n1+ n2- 1) degrees of freedom (n1+ n2- 2) degrees of freedom n1- n2+ 2 (n1+ n2) degrees of freedom