Finally, we observed that the distributions of a number of traits in some of the sites were highly skewed and differed across samples (results available on request). This is not surprising, as there was likely to be considerable heterogeneity across sites in item definition, rater training, patterns of help-seeking (affecting age of onset), treatment setting (e.g. ambulatory, institutionalized, etc.) as well as other unobserved patient- or rater-dependent factors. Because this could considerably inflate genetic analyses, we standardized all traits within site, to have mean=0 and SD=1 (treating the individual MGS sites separately).
