In Testing the Difference Between Two Population Means Using Two $\overline { \mathrm { x } } _ { 1 } - \overline { \mathrm { x } } _ { 2 }$

Multiple Choice

In testing the difference between two population means using two independent samples,we use the pooled variance in estimating the standard error of the sampling distribution of the sample mean difference $\overline { \mathrm { x } } _ { 1 } - \overline { \mathrm { x } } _ { 2 }$ if the:

A) sample sizes are both large.
B) populations are at least normally distributed with equal variances.
C) populations are non-normal with unequal variances.
D) All of these are required conditions.