Because the individual symptoms assessed differed substantially across sites, we sought to achieve across-site commonality at the level of symptom factors. We therefore constructed quantitative traits common to all instruments and sites, and which could be used as phenotypes of interest in genetic studies. Our approach was stepwise, involving initial exploratory factor analysis of the each instrument in each individual site, followed by harmonization of the different sites by selecting prominent items and factors, and finally, calculation of factor scores using confirmatory factor analysis in each site separately, as follows.
