CBO's analyses of the distribution of household income rely on the Census Bureau's Current Population Survey (CPS) for information about receipt of government transfers, particularly means-tested transfers. CPS respondents underreport their receipt of those transfers, and that underreporting has increased over the past few decades. This presentation shows how CBO adjusts for the underreporting of means-tested transfers in its distributional analyses.
CBO has also released a set of CPS extracts and Stata code to allow researchers to replicate the agency's results for 1979 through 2018. Those data and code are available on GitHub.