Cassandy is a third year facilitator and is assigned to compare the academic performance of the boys and girls in the third year level in Math, Science, and English. There are 8 sections ranging from 41 to 43 students per section. In order to get a sample, he grouped the boys separately from the girls in each section then got the top 2, the middle 2, and the bottom 2 boys and girls per section. From the sample, he got the general weighted average per subject of the boys separately from the girls and then compared the results. Identify the sampling procedure used in this situation and explain your answer.