Reliability of Survey Estimates

The standard error of an estimate is primarily a measure of the sampling variability that occurs by chance because only a sample is surveyed, rather than the entire universe. Because the NHAMCS is a sample survey, users should be aware of the reliability or unreliability of survey estimates, particularly the smaller estimates. NCHS considers an estimate to be reliable if it has a relative standard error of 30 percent or less (that is, the standard error is no more than 30 percent of the estimate). It should be noted, too, that estimates based on fewer than 30 records are also considered unreliable, regardless of the magnitude of the relative standard error.

The relative standard error of an estimate is generally related in a curvilinear fashion to the size of the estimate. Therefore, approximate relative standard errors for NHAMCS estimates have been calculated based on a generalized variance curve that is derived using SUDAAN software. These tables can be downloaded as part of the Public-Use Documentation files.