Why did NCS-R have two parts?

The National Comorbidity Survey Replication (NCS-R) Part 1 included a core diagnostic assessment of all 9,282 respondents and Part 2 was administered only to 5,692 of the 9,282 Part 1 respondents, including all Part 1 respondents with a lifetime disorder plus a probability subsample of other respondents. See NCS-R Part 1 and 2 Sample for more information. See Final Weights and Special Analysis Considerations for Weighted Analysis in the Weighting section of About CPES for an explanation of the two parts and the implications for weighting analyses.