Regardless of how items are arranged or how domains are grouped, it is generally agreed that the content validity of PROs can be judged only by the persons or populations being assessed. Thus, the extent to which the domain of interest is comprehensively sampled by the items or questions in the measure can only be judged by representatives of the target population. If the target population is unable to speak for themselves, proxy judgments are sometimes considered acceptable, particularly if supported by rigorously controlled observational studies with inter-rater reliabilities. Before assuming that people cannot speak for themselves, however, they should be asked and every effort should be made to communicate with them directly. Proxy responses are not PROs.