One of the purposes of NHIS data is to study relationships between income and health and to monitor health and health care for persons at different income levels. However, non-response rates are high for questions on total family income and personal earnings from employment in the previous calendar year. To obtain estimates of these variables for all survey participants, the National Center for Health Statistics created variables with values imputed for missing data for 1997 forward, using multiple-imputation methodology. The NHIS public use files with multiply imputed data consist of five files (and thus five versions of variables containing imputed values for missing data), one for each set of imputed values, to allow the assessment of variability due to imputation. For more information on the imputation methodology, see EMPSTATIMP1.

Before using the imputed income and earnings variables, researchers are strongly advised to read the NCHS documentation on imputed income, such as 2018 Imputed Family/Personal Earnings Files and "Multiple Imputation of Family Income and Personal Earnings in the National Health Interview Survey: Methods and Examples".