professor rodriguez is interested in studying the relationship between doing homework and academic achievement. in august, dr. rodriguez has his students report their prior spring gpa (as a measure of academic achievement) and estimate how many hours they spent working on homework during a typical week in the spring semester. in december, dr. rodriguez measures the same variables again (the estimated number of hours spent doing homework during a typical week in the fall semester and their fall gpa). what type of study did professor rodriguez conduct?