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Published online by Cambridge University Press: 01 August 2014
With the exception of disorders related to gestation and birth, twins are usually found to be medically representative of the general population; diseases occur among twins with the same frequency as among singletons. As a corollary, in a large series of twins, if members of each pair are assigned randomly to two groups, A and B, both groups will be representative of the population. For example, if a disease occurs in 1% of the population, then we can expect it in 1% of twin-group A and in 1% of twin-group B.
Tab. I diagrams a hypothetical population of 15000 twin pairs in which 1% of all persons have the disease in question. Twins A include, in the left column, 150 sick persons. Each of these has a partner, twin B, who may be either sick or well, and so the total number of pairs in the left column is 150. The figures are the same when the table is read down or across; that is, the table is symmetrical with respect to twins A and twins B because assignment to A and B was random.
* The methods recommended here correct only for sampling; i.e., from sample data they yield consistent estimates of statistics that would be obtained if the whole population were studied by the same methods. Z. Hrubec has shown (paper in preparation) that when incomplete ascertainment is uniform and hence equivalent to incomplete penetrance, the proband concordance rate is reduced in proportion to ascertainment. However, in this case the ratio of observed and expected proband concordance is the same as the true ratio. When ascertainment is complete in partners of index cases, index cases would of course be used alone in the calculations, and the correct estimates would result.
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