To match files, each file must have a unique identifier and the identifiers used in the different systems must be compatible. This is often not the case. For example, TANF record identifiers are based on families, while child welfare records are identified by the names of the child's parents. Neither vital statistics nor substance abuse records have unique identifiers, in the latter case because of confidentiality concerns. Several states assign their own identification numbers to individuals so they can track the individuals across a variety of systems.