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 2008 Imputed Family/Personal Earnings Files and "Multiple Imputation of Family Income and Personal Earnings in the National Health Interview Survey: Methods and Examples".

Specifically, the original values distinguished between: a) cases that were not imputed, but were instead based on reported income; b) cases that were imputed, in which respondents supplied no income information; c) cases that were imputed with "2-category income reported" (i.e., income reported to be above or below $20,000); and d) cases that were imputed, with "44-category income reported." (A small number of cases were also categorized as "indefinable," for instances in which all co-resident family members were under age 18.)

Beginning in 2007, the source variables for IMPOVFLAG2 in the original public use files distinguished between: a) cases that were not imputed, but were instead based on reported income; b) cases that were imputed, in which respondents supplied no income information; and c) cases that were imputed with income "reported in categories." (Again, a small number of cases were also categorized as "indefinable," for families in which all co-resident members were under age 18.)

The first digit in composite coding covers broad categories available across all years, while trailing digits preserve detail present in only some years. Specifically, in IHIS, cases in IMPOVFLAG2 that are not imputed are coded 0; cases that are imputed with no income information begin with a 1; cases that are imputed with some income information share a common first digit of 2; and "indefinable" cases begin with a 9. The differences between the various kinds of imputation with some income information are indicated in codes 20 (imputed, some income reported in 2007-2008), 21 (imputed, 2-category income reported in 1997-2006), and 22 (imputed, 44-category income reported in 1997-2006).

Composite coding provides a mechanical solution to the change in imputation methods and codes in IMPOVFLAG2. Across all years, researchers can combine cases sharing a common first digit to produce a standard 4-category system of "not imputed," "imputed with no income information,""imputed with some income information," and "indefinable." Even then, the underlying substantive difference in collecting income information persists; this in turn somewhat limits the comparability of IMPOVFLAG2 and the complementary variable POVIMP2.