All estimates derived from a sample survey are subject to sampling and nonsampling errors.
Sampling errors occur because observations are made on a sample, not on the entire population. Estimates based on the different possible samples of the same size and sample design could differ. The relative standard errors, which are a measure of the sampling error in the estimates, are calculated as part of the survey's estimation process. Nonsampling errors in the estimates can be attributed to many sources, e.g., inability to obtain information about all cases in the sample, mistakes in recording or coding the data, definitional difficulties, and so forth. Although not measured, nonsampling errors will always occur when statistics are gathered.