These estimates are based on ERIU tabulations of data collected by the U.S. Census Bureau through the 2004 Current Population Survey (CPS) Annual Social and Economic Supplement (formerly called the Annual Demographic Supplement, or the March Supplement). CPS estimates of the uninsured are intended to represent persons who are uninsured throughout the calendar year. A person is considered insured if they were covered at any time during that calendar year; individuals insured for only one month are counted as insured. CPS estimates of the uninsured should be interpreted with caution. Respondents are asked to report on a time period that begins roughly 15 months prior to the time of the interview. Some respondents are likely to have trouble remembering back this far, while others may be confused by the time frame and respond instead about current coverage status. Studies that have examined and compared CPS estimates with data from other surveys, such as the Survey of Income and Program Participation (SIPP) and the Medical Expenditure Panel Survey (MEPS), suggest that CPS estimates of the uninsured are much closer to “point in time” than true full-year estimates.

For further discussion of issues related to counting the uninsured see "Counting and Characterizing the Uninsured," (Adobe PDF) by Pamela Farley Short, and ERIU Research Highlight 1: A Revolving Door: How Individuals Move In and Out of Health Insurance Coverage (Adobe PDF)

Notes on CPS Data Source and Variables (PDF)