A type of research whose aim is to measure the level of association, if any, between two or more variables. Methods can include observing people, carrying out surveys, or analyzing historical records.

Example: A researcher is currently examining old prison records to gather data about prisoners’ education levels. She hypothesizes that there is a negative relationship between number of years spent in prison and education level; that is, the more years a person has spent in school, the less likely he or she is to have spent a significant amount of time in prison.