Scrivener et al. (2001) tested the hypotheses that the risk of asthma is reduced by intestinal parasites or hepatitis A infection, and increased by exposure to dust-mite allergen in urban and rural areas of Ethiopia. They used a case-control design, comparing people who reported wheeze in a survey with a sample of those who did not. They reported that the risk of wheeze was independently reduced by hookworm infection by an odds ratio of 0.48 (95% CI 0.24-0.93, p=0.03), increased in relation to the level of house dust mite measured in bedding (odds ratio per quartile 1.26 [1.00-1.59], p=0.05), and was unrelated to hepatitis A seropositivity or cholinesterase concentration. In the urban population, house dust mite skin sensitisation was more strongly related to wheeze (9.45 [5.03-17.75]) than in the rural areas (1.95 [0.58-6.61], p for interaction=0.017).
1. Why are the results reported in the form of odds ratios?
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2. What is an interaction? How would they test for an interaction between urban-rural living and house dust mite skin sensitisation?
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3. The risk of wheeze increased in relation to the level of house dust mite measured in bedding, the odds ratio per quartile being 1.26. What is a quartile?
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4. What do they mean by the odds ratio per quartile? Warning, this is really tough!
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Scrivener S, Yemaneberhan H, Zebenigus M, Tilahun D, Girma S, Ali S, McElroy P, Custovic A, Woodcock A, Pritchard D, Venn A, Britton J. (2001) Independent effects of intestinal parasite infection and domestic allergen exposure on risk of wheeze in Ethiopia: a nested case-control study. Lancet 358, 1493-1499.
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Last updated: 31 July, 2006.
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