Читать книгу Understanding Clinical Papers - David Bowers - Страница 23

A more satisfactory way of investigating cause and effect is to concentrate on a clinical scenario in which the characteristic that you suspect might be a risk factor and the outcome can only have arisen in that order. Consider for a moment smoking and lung cancer: it is plain that contracting lung cancer cannot have led someone to become a long‐standing heavy smoker.

Notice though that it is the pre‐existence of heavy smoking that defines the difference between this example and the cross‐sectional example above. If a researcher wanted to know whether high blood pressure made stroke more likely and chose to measure blood pressure in two groups of patients, who were and were not victims of stroke, then any finding that hypertension was more prevalent in stroke patients might be a consequence of the stroke rather than a contributory cause. If, on the other hand, each set of patients had previously had their blood pressure recorded some years before, then the finding that stroke patients had a past excess of hypertension might very well point to high blood pressure being a risk factor for stroke.

Case–control study is the label applied to a study such as the one just mentioned, about high blood pressure and stroke. The group of people with the condition are called the cases and they are compared with another group who are free of the disease and are called the controls. The comparison to be drawn is the exposure of each of the two groups to a supposed risk factor: were the cases more often exposed to the risk than were the controls? For further discussion of cases and controls, see Chapters 16 and 17.

In the study in Figure 6.3, mental health researchers undertook a case–control study to help to determine the role of cannabis in the incidence of psychosis. They recruited, from 11 sites across Europe and Brazil, 901 consecutive patients with first‐episode psychosis. They also recruited 1237 controls – people who did not have psychosis – from the same geographical catchment areas, using various sampling strategies involving random selection, from lists of postal addresses and from general practitioner lists. Using a modified version of the Cannabis Experience Questionnaire, they asked all participants about their use of cannabis and other recreational drugs. They found, for example, that there was a much higher risk of psychosis among people who reported daily cannabis use than among those who had never used the drug (after adjusting for various possibly confounding factors): the adjusted odds ratio was 3.2 – see Chapter 28 for an explanation of odds ratios in case–control studies, and Chapter 17 for material about confounding.

Figure 6.3 A case–control study examining the relation between cannabis use and onset of psychosis.

Source: From Di Forti et al. (2019), © 2019, Elsevier.