Clinical research sometimes involves a single group and requires no comparison group. In conducting such a study, which is often encountered in surgical settings, the preoperative, immediate operative, and follow-up measures are used to determine the effectiveness of a surgical procedure. In other clinical settings, a pretest measure is obtained from a single sample before intervention, and the same instrument is used to obtain information from the same sample after intervention. These tests allow one to infer conclusions about the wider population from the data on selected individuals (sample).