Treasury's Office of Tax Analysis (OTA) and its counterparts in the Congress, the Joint Committee on Taxation (JCT) and the Congressional Budget Office (CBO), are responsible for providing policy makers with economic analyses of the effects of current and proposed federal tax law. Typical analyses address such issues as the effect of the law on government receipts, the effect on the distribution of the tax burden, and the effect on incentives to work, save, and engage in other economic activities. Until recently, these analyses have been based almost exclusively on data from "cross-section" samples, or models based on such data. A cross-section sample is one drawn from a population at a point in time, such as a sample of all tax returns filed in a particular year. Such samples are designed to accurately represent the characteristics of the population for the year or other point in time covered by the sample.