This categorical variable (ZIPINC) indicates the median household income of the patient's ZIP Code of residence. The median income values are 1999 estimates derived from projections from 1990 Census values for block groups. The categories are defined so that the maximum for category 1 ($25,000) is approximately 150% of the 1999 poverty level and the boundary between the second and third categories ($35,000) is approximately the national median household income.

Beginning in 2003, another categorical variable (ZIPINC_QRTL) provides a quartile classification derived from annual ZIP Code-demographic data obtained from Claritas. Because these estimates are updated annually, the value ranges for the ZIPINC_QRTL categories vary by year.

To protect patient confidentiality, precautions are taken to avoid indirectly identifyiing ZIP Codes. ZIPINC values have been suppressed for all ZIP Codes with populations below a minimum threshold. Also, when only one ZIP Code was represented in a particular category in ZIPINC for a state, ZIPINC was set to missing.

ZIPINC is missing (.) when the patient's ZIP code was missing, did not exist in 1999, was invalid in 1999, or outside of the United States.

Prior to 1998, information on the median household income for a patient's ZIP Code was retained in two data elements ZIPINC4 and ZIPINC8.