when i was doing my master thesis in graduate school, i researched on the problems of college students and how this affected their academic performance. so from there, i had to have an instrument in knowing their various problems (i used the mooney problem checklist) and asked the college a copy of their midterm grades.

now, in ANALYZING how the latter affects the former research variable, i had to do correlation and analysis of variance (ANOVA). usually, a statistician does this for you. so you see, this is also why a research team is composed of many people, not just one. you have the main or principal researcher, the data gatherers or junior researchers, and the statistician.