This codebook documents version 5.1 of the SIPP Synthetic Beta (SSB). The SSB is a set of files containing individual-level data synthesized from linked survey and administrative data. The SSB is produced by the US Census Bureau as part of a joint project with the Social Security Administration (SSA), and the Internal Revenue Service (IRS). The goal of the project is to make some of the benefits of linked survey and administrative data available to researchers outside of restricted-access Census Bureau facilities in a manner that protects the confidentiality of the underlying data.