Information on important data elements is frequently missing or recorded inaccurately. For example, demographic data are often unreliable because their accuracy was not important to program success. Historically, data on the reasons for welfare case closure have not been well documented. Wage record data provided by employers on migrant workers is notoriously unreliable because they often report multiple workers as having the same Social Security number. In addition, file contents tend to change over time so comparative data are difficult to find.