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Another way of identifying any relation between cannabis and onset of psychosis would have been to compare the eventual outcome for people who did or did not use cannabis – a cohort study – sometimes termed a cohort analytic study, thereby emphasizing the comparative (or analytic) objective of the investigation. In this hypothetical example, the subjects of the research would have been divided by the researcher according to whether they were cannabis users or not. In the earlier, real, case–control study the study participants were divided by whether they had the disease (psychosis in this case) or not; the designs are quite different.

In a real example of a cohort analytic study shown in Figure 6.4, researchers looked into whether open rather than laparoscopic surgery – in the abdomen or pelvis – is a risk factor for later readmission as a consequence of surgical adhesions. Adhesions are a kind of scar tissue that develop in most patients who have abdominal or pelvic surgery, and they are a major cause of long‐term morbidity. The researchers used National Health Service data from Scotland to determine whether each woman who had an abdominal or pelvic operation had been readmitted as a consequence of a disorder caused by adhesions over the next five years and, in each case, whether the original surgery was by laparoscopic or open procedure. The likelihood of a readmission due to adhesions was compared between the women who had laparoscopic and open surgery.

Figure 6.4 A retrospective cohort analytic study examining the relation between readmission to hospital for disorders directly related to adhesions and earlier open or laparoscopic abdominal surgery.

Source: From Krielen et al. (2020), © 2019, Elsevier.