A researcher studies how creativity relates to intelligence by measuring both constructs in the same sample of students. In addition, she also measures openness to experience to control for it in her analyses and rule it out as a third variable. What kind of research design has the researcher used?A researcher is interested in the effectiveness of a new study skills program. He pre-selects a sample of new university students whose grades are especially low. He then has them participate in the study skills program over a two-month period. At the end of the two months, their grades are higher, on average. What threat to validity should the researcher be concerned about