Finally, our results have potential implications for the optimal ways in which studies of EPs and PDs can be conducted. For example, given awareness of the problems of error, researchers might modify their protocols to improve the reliability of their EPs by including either multiple ratings or multiple measurements over time. Both strategies have been shown to substantially increase the heritability for PDs that are assessed with significant error.18,23 Furthermore, multiple measurements, longitudinal designs and/or the incorporation of specific risk genes in statistical models, have the capacity to clarify the causal interrelationships for EPs and PDs.24
