Carlos asked students if they belonged to a sorority or frat…
Carlos asked students if they belonged to a sorority or fraternity, and asked them about their current GPA. He discovered that the GPAs of those people in sororities and fraternities are higher than those of people who are not involved in the Greek system. He presented his findings to the dean of students, saying that there is a causal relation between grades and group membership in sorority or fraternity. What principle of research methodology is Carlos violating?