Pooling is used in calculating the standard error for the difference between means when the population standard deviations are unknown, for which of the following reasons?
- because the weighted average of the two sample variances gives a better estimate of the common population variance
- because the weighted average of the two population variances gives a better estimate of the common sample variance
- because the variances in the populations are assumed equal
- because the null hypothesis usually shows there is no difference between the means