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To the Editor:
In the article by Maher et al.1 on sibling pairs with PD the authors found that the intraclass correlation of ages at onset of PD between sibling pairs (r = 0.56) was larger than the correlation between the corresponding years of onset (r = 0.29). The authors concluded that this observation supports a genetic component for PD. Although this may well be the case, there are some statistical problems, mostly associated with the study design, that preclude drawing such a conclusion from this study.
First, this is a cross-sectional study (data collection occurred between September 1997 and July 2000), which forces the distribution of year at onset to be highly skewed. In other words, cases of recent diagnosis (e.g., between 1990 and 2000) will tend to predominate, particularly in a degenerative disorder such as …
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