This article examines two methodological issues regarding ways of obtaining and analyzing outcome data for people with intellectual disabilities: (a) self-report and proxy-report data and (b) analysis of population-based data sets. Some people with intellectual disabilities have difficulties with self-reporting due to problems of understanding and communication. However, there are serious doubts about the validity of proxy data for subjective issues. One important challenge with secondary analysis of population-based data sets is the difficulty of accurately identifying survey participants with intellectual disabilities. In both areas examined, it is important to recognize these constraints when interpreting research based on such data.