ASSOCIATION WITHIN TWIN PAIRS FOR A DICHOTOMOUS TRAIT

We propose an odds-ratio measure of twin association for a dichotomous trait. This odds ratio can be estimated without arbitrarily specifyin g an index twin and without estimating additional nuisance parameters. Tests of association and confidence intervals maybe computed easily, in contrast to those proposed previously [Donner et al. (1995) Genet E pidemiol 12:267-277] for a correlation measure of association. For tes ting homogeneity of association in two samples of twins, monozygotic a nd dizygotic, we propose a large-sample test and an exact test, both b ased on the odds-ratio parameterization. Large-sample tests of homogen eity are slightly anti-conservative when some cell counts are small, a nd the exact test may be preferred in these situations. We also propos e a log-linear parameterization useful for modeling more complex data sets. (C) 1996 Wiley-Liss, Inc.