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The research shown here investigating the possible relation between open (rather than laparoscopic) surgery and adhesions‐related readmission is an example of a retrospective cohort study. The researchers looked back into the Scottish record‐linkage data to determine the presence or absence of the risk factors of interest (open surgery versus laparoscopic surgery) and examined the occurrence of adhesions‐related readmission to hospital over the years between the operation and some recent time. More often, cohort analytic studies are prospective in their design. In one example (Figure 6.5), Canadian researchers recruited a consecutive series of over 300 women who had a child delivered at the hospital. As it happened, around half of the women opted for epidural anaesthesia, allowing a comparison between them and women whose labour was assisted by alternative forms of pain relief – to determine whether new back pain had higher incidence among those who had received an epidural.

Figure 6.5 Prospective cohort analytic study examining the relation between post‐partum back pain and epidural anaesthesia during labour.

Source: From Macarthur et al. (1995), © 1995, BMJ Publishing Group Ltd.

You may have noticed one prominent feature that distinguishes case–control from cohort designs: case–control studies ask the question by looking backwards (asking whether more cases than controls had been exposed to some risk factor), while cohort analytic studies ask their question by looking forwards (asking whether more people who were exposed to the risk, than people who weren't exposed, developed the outcome in question). But you need to be careful with your thinking and your terminology in relation to the words retrospective and prospective because, as we have seen, it is possible to ask the (forward‐going) question of the cohort study by starting now and looking ahead (Figure 6.5), or by starting with the presence and absence of the risk factor some time previously and following up until lately (Figure 6.4). Put another way, case–control studies are retrospective but cohort studies may be described, quite properly, as either prospective or retrospective.