Figure 3.1 Prevalence of self-reported cough first thing in the morning in Derbyshire schoolboys, by their own and their parentsí cigarette smoking (data from Banks et al. 1978).
Most diseases are not suited to this simple crosssectional approach, because they are rare events. For example, lung cancer accounts for 9% of male deaths in the UK, and so is a very important disease. However the proportion of people who are known to have the disease at any given time, the prevalence, is quite low. Most deaths from lung cancer take place after the age of 45, so we will consider a sample of men aged 45 and over. The average remaining life span of these men, in which they could be diagnosed with lung cancer, will be about 30 years. The average time from diagnosis to death is about a year, so of those who will contract lung cancer only 1/30 will have been diagnosed when the sample is drawn. Only 9% of the sample will develop lung cancer anyway, so the proportion with the disease at any time is 1/30 × 9% = 0.3% or 3 per thousand. We would need a very large sample indeed to get a worthwhile number of lung cancer cases.
Cross-sectional designs are used in clinical studies also. For example, Rodin et al. (1998) studied polycystic ovary disease (PCO) in a random sample of Asian women from the lists of local general practices and from a local translating service. We found that 52% of the sample had PCO, very high compared with that found in other UK samples. However, this would not provide a good estimate for Asian women in general, because there may be many differences between this sample, such as their regions of origin, and Asian women living elsewhere. We also found that PCO women had higher fasting glucose levels than non-PCO women. As this is a comparison within the sample, it seems plausible to conclude that among Asian women, PCO tends to be associated with raised glucose. We cannot say whether PCO raises glucose or whether raised glucose increases the risk of PCO, because they are measured at the same time.
Rodin, D.A., Bano, G., Bland, J.M., Taylor, K., Nussey, S.S. (1998) Polycystic ovaries and associated metabolic abnormalities in Indian subcontinent Asian women. Clinical Endocrinology 49, 91-99.
Adapted from pages 31–32 of An Introduction to Medical Statistics by Martin Bland, 2015, reproduced by permission of Oxford University Press.
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