Tuesday, March 15, 2011

DEFINING THE DATA PRODUCTS

Access Content Map for ACS Guide. FAQ's. Introduction to ACS. Sample Data Products

A section of Data Products by Scott Boggess (U.S. Census Bureau) and Linda A. Jacobsen (Population Reference Bureau)

ACS data products were initially designed to be comparable to Census 2000 long-form sample products. However, based on extensive input from data users, the ACS data products have been redesigned and expanded, and additional tables have been added to reflect new content in the ACS that was not included in the Census 2000 long form. There are some differences between the data products provided for the one-year, three-year, and five-year ACS data. Table 1 provides a comparison of the ACS data products with the Census 2000 long-form sample products. These data are available through American FactFinder (AFF) on the Census Bureau’s Web site, accessible either directly or as downloadable files.

Table 1: Comparison of Census 2000 and ACS Data Products

The ACS was fully implemented in 2005, and one-year ACS data products are available for each year thereafter. Three-year ACS data products started in 2008, with the release of the 2005–2007 period estimates and subsequent releases for 2006–2008 and 2007–2009. The first five-year ACS data products for 2005–2009 were released at the end of 2010. Annual releases of one-year, three-year, and five-year ACS data products are planned for 2011 and each year thereafter.

ACS data products can be divided into two broad categories: tabulated data products, and microdata files. The tabulated data products contain precalculated estimates of frequently requested totals, percentages, means, medians, and ratios. The microdata files, known as Public Use Microdata Sample (PUMS) files, contain a sample of the individual person and housing data and can be used to produce estimates not already published in the tabulated data products.

One important difference between ACS data products and Census 2000 data products is the inclusion of 90-percent margins of error (MOE) for all estimates in ACS tabulated data products. The MOE is a measure of the precision or reliability of an estimate. The larger the margin of error for an estimate, the less reliable or precise is that estimate. These margins of error can also be used to calculate 90-percent confidence intervals, which indicate that data users can be 90-percent confident that the true population value will usually fall within the range defined by the confidence interval. The margins of error in ACS tabulated data products provide an important tool to help data users understand the reliability of ACS estimates and to draw appropriate conclusions from the data. Measures of reliability are not provided in ACS PUMS files but can be calculated using either a standard formula or the replicate weights that are included in the files.

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