Overview > Data Sources & Limitations

Data Sources and Limitations

Most of the data in the POVERTY BASICS section of the website come from two surveys implemented by the U.S. Census Bureau – the American Community Survey and the Current Population Survey (Annual Social and Economic Supplements).  The Current Population Survey is primarily designed to generate national level estimates of poverty rates, income and earnings.  The American Community Survey, because of its larger sample size, is the best source for sub national estimates of poverty and income, and contains data for states, counties and cities (with a population of at least 65,000 people), as well as national estimates.   The two surveys may produce slightly different results because both the populations surveyed and the timing of the surveys vary.  For more information about these surveys and other source of data on poverty and income, click here.

Because the data come from surveys, they are based on a sample and are subject to sampling variability.  Each estimate has a certain degree of uncertainty due to sampling variability, and this uncertainty is represented by the margin of error.  A 90 percent margin of error (the standard used by the Census Bureau) can be understood as reflecting a 90% probability that the interval defined by the estimate minus the margin of error and the estimate plus the margin of error contains the true value.  Because of sampling variability, readers should be cautious about assuming that any two estimates are significantly different from each other.  Statistical tests can be conducted that determine whether a difference is significant. 

The U.S. Census Bureau publication on the American Community Survey provides more information about this survey and explains how the reader can test for statistical significance: http://www.census.gov/acs/www/Downloads/ACS/accuracy2005.pdf

Similarly, the U.S. Census Bureau publication “Source and Accuracy of Estimates for Income, Poverty and Health Insurance Coverage in the United States: 2005” provides background information regarding the Current Population Survey data: http://www.census.gov/hhes/www/income/p60_231sa.pdf