For more details on the purpose and methodology of imputation used in the NHIS, see EMPSTATIMP1.

Before using the imputed income and earnings variables, researchers are strongly advised to read the National Center for Health Statistics 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".

For 1997-2006, the category labeled "family income reported" (IHIS code 0) means, according to a note in the codebook for those years, that family income was not imputed, but rather "is based on reported data (either exact income or 44-category income group)."

For 2007 forward, this same non-imputed "family income reported" category means "family income group is based on reported income (exact income)."

For 1997-2006 and for 2007 forward, the category labeled "imputed, no income information reported" (IHIS code 1) has the same meaning in all years.

For 1997-2006, the category labeled "imputed, some income information reported" (IHIS code 2) means "11-category income group was unknown (2-category income information was reported)." In other words, in 1997-2006, imputation for category 2 was based on the information that family income was above or below $20,000.

For 2007 forward, the category labeled "imputed, some income information reported" means, according to a note in the codebook for those years, that "family income group is based on imputed family income because income only range was reported."

For 1997-2006, when respondents refused or did not know their family income, interviewers first asked whether the amount was above or below $20,000 and then asked them to pick the appropriate category from a set of grouped income intervals.

Beginning in 2007, interviewers probed for family income information using an unfolding bracket methodology. This method asked a series of closed-ended income range questions (e.g., "is it less than $50,000?"), constructed so that each successive question established a smaller range for the amount of the family's income. This shift in methodology is reflected in the changing meaning of two categories (IHIS codes 0 and 2) for IMPYFAMFLAG4.