Self-reporting
This is a common and economical, if not always reliable, way that researchers gather information about study subjects; the idea is that subjects describe or report their own behavior to researchers. In a diary study, subjects record data during specified time periods. An example of a diary study would be the Nielsen ratings, in which families record their TV viewing habits in a daily diary. Self-reporting can take the form of an interview or series of interviews. Finally, a questionnaire is a list of questions that people read and answer. Questionnaires are less flexible than interviews, but are more standardized and less influenced by interactions between an interviewer and subject. When you see that a study is based largely on self-reported data, this should serve as a red flag. You should look at this kind of data with a skeptical eye, asking yourself whether or not you would be completely honest or objective in answering the kind of questions researchers asked in this particular study.