Читать книгу Understanding Clinical Papers - David Bowers - Страница 27

Compared with cohort studies, case–control studies are cheap and cheerful. First, they can be completed without waiting for the outcome to develop. Second, there is no need for enormous numbers of study subjects who do not develop the outcome; because in a case–control study it is already clear who has the outcome, only a convenient number (needed for reasonable statistical precision) of these 'controls' is needed. In a cohort study, however, there is no way of knowing who will get and not get the outcome – so everyone has to be included until the study end‐point reveals who has developed the condition so far. For a great many relatively uncommon outcomes the case–control study is favoured for just these reasons. Examples of case–control studies include investigating whether deep venous thrombosis is related to taking the oral contraceptive pill, and whether depressive illness is related to adverse life events and difficulties.

On the other hand, there are serious shortcomings with case–control studies. They are prone to extra biases – in particular those concerned with recollection of past events. Suppose, for example, that a study hypothesis is that an adult condition such as motor neurone disease is predisposed to by certain kinds of childhood infections. It is likely that in an effort to understand why one has such an illness, sufferers with motor neurone disease may recall more such illnesses than controls who have no pressure to make such an effort. It is very difficult to avoid such bias.

Second, it is usually difficult to select a wholly suitable control group for a case–control study. In a case–control study concerned with smoking and lung cancer, for example, should those without lung cancer be people with other cancers, or patients with other respiratory conditions, or other patients at the hospital without chest disorder, or general practice cases (and should they exclude those with chest problems), or members of the general public who are not drawn from healthcare contacts at all? None of these possible groups is wholly suited or unsuited; in the end, the findings will often be hard to interpret whichever control group is selected (sometimes case–control studies use more than one control group for just such reasons). Put another way, the control group in a case–control study is intended by the researcher to represent, as if it were a small random sample, the great many people in the population who have not developed the outcome. Unfortunately, in practice, they are instead a specifically targeted group – easily identified and included in the research – but not actually providing the representation of the non‐cases in the population that the researcher ideally wants.

Cohort analytic studies avoid the worst aspects of the above two problems. First, because exposure is assessed at the start of the study, it is not subject to false remembering later. Second, assessment of exposure to risk often provides sensible exposed and unexposed groups, and avoids uncertainty over which comparison group to choose.

Both these kinds of analytic observational study are greatly prone to confounding. This matter is discussed in Chapters 17, 34, and 35.