What statistical tests should be done if I want to check how a teaching intervention affects student achievement? I have a control group and an experimental group, each group is composed of 16 students who were divided into two groups based on their computed average grade in their Mathematics class - there are equal numbers of low, average, and high performing students in each group. I have their pre-test scores and post-test scores and an independent t-test showed that the pre-test scores of the two groups do not significantly differ. From here, how should I proceed to check and compare student achievement? Thanks in advance!