Does anyone know why we use ttest for equal variances instead of a paired t-test in bioequivalence study?

Question

For bioequivalence studies of pharmaceuticals.

Answer 1

I dont know what a bioequivalence study is, but you only use a paired t-test on paired observations e.g. two blood pressure observations (such as before and after treatment) on the same patient.
You use the unpaired t-test on unpaired data e.g. two different groups of individuals' blood pressure observations. You use the test with equal variance if you can assume that the populations ( the two groups of patients) have equal variance.
Hope this helps!
I'm a bit rusty, but I think you use the equal variance unpaired test when you can - as opposed to an unequal variance unpaired test - as this gives a lower total variance and hence a more powerful test.