Statistics compiled for the SOI studies are generally based on stratified probability samples of income tax returns or other forms filed with the Internal Revenue Service (IRS). The statistics do not reflect any changes made by the taxpayer through an amended return or by the IRS as a result of an audit. As returns are filed and processed for tax purposes, they are assigned to sampling classes (strata) based on such criteria as: industry, presence or absence of a tax form or schedule, and various income factors or other measures of economic size (such as total assets, total receipts, size of gift, and size of estate). The samples are selected from each stratum over the appropriate filing periods. Thus, sample selection can continue for a given study for several calendar years—3 for corporations because of the incidence of fiscal (noncalendar) year reporting and extensions of filing time. Because sampling must take place before the population size is known precisely, the rates of sample selection within each stratum are fixed. This means, in practice, that both the population and the sample size can differ from those planned. However, these factors do not compromise the validity of the estimates. Statistics of Income General Sampling Methodology and Data Limitations (PDF - Get Adobe Acrobat Reader) This article discusses typical sampling procedures used in most Statistics of Income (SOI) programs. Aspects covered briefly include sampling criteria, selection techniques, methods of estimation, and sampling variability (coefficients of variation (CV)). Some of the nonsampling error limitations of the data are also described, as well as the tabular conventions employed. Sampling Methodology and Data Limitations, by Study Additional information on sample design and data limitations for specific SOI studies can be found in the link above. More technical information is available, on request, by writing: Internal Revenue Service, (RAAS:S) Attn: SOI Director 1111 Constitution Ave., NW K-Room 4160 Washington, DC 20224-0002 Statistics of Income Research in Statistical Methodology Follow the link above to find research papers written by members of SOI covering topics such as sample design and models, panel design and analysis, data disclosure and data sharing research, and more.