Suppose you want to test whether a drug would produce a better outcome than non-treatment in a trial involving humans. Enumerate as many possible ways as you can where the observed sample mean differences do not reflect the true effect of the drug. You need to explain each of these ways in a few sentences, giving enough details to make clear what the underlying problem is. You may give real examples if you wish.
Y represents response under control.
Suppose both are normally distributed with respective means m x and m y with the same standard deviation s . If the standard deviation is 1 and each of the sample sizes is 20, calculate the standard error of the difference.