It is frequently stated that about 40 million Americans lack health insurance. That estimate, however, overstates the number of people who are uninsured all year. The Congressional Budget Office (CBO) estimates that between 21 million and 31 million people were uninsured for the entire year in 1998-the most recent year for which reliable comparative data are available. Since then, the number who are uninsured all year probably has not changed substantially, given historical trends. The uninsured population is fluid, with many people gaining and losing coverage. For example, between half and two-thirds of the people who experienced a period of time without insurance in 1998 had coverage for other portions of the year. The commonly cited estimate of 40 million uninsured comes from the Census Bureau's Current Population Survey (CPS). Based on a large nationally representative sample, the CPS has been collecting data on health insurance status since 1980.