Prior to 1998, there are two categorical median income ZIP Code data elements derived from the patient's ZIP Code, one with four categories (ZIPINC4) and another more specific data element with eight categories (ZIPINC8). ZIPINC4 and ZIPINC8 are based on median household income. Beginning in 1998, there is only one data element for median household income of the patient's ZIP Code: ZIPINC from 1998-2002 and ZIPINC_QRTL beginning in 2003.

To protect patient confidentiality, precautions are taken to mask the more specific eight-category data element. When less than three ZIP Codes were represented in a particular category in ZIPINC8 for a state, ZIPINC8 was set to missing and only ZIPINC4 is reported for that state. Otherwise both ZIPINC8 and ZIPINC4 are reported. The categories for ZIPINC4 were designed specifically so that no category would represent less than three zip codes in any state.

For example, if in state A only 2 ZIP codes fall into the $25,001-30,000 median income range, then ZIPINC8 is missing (.) for all ZIP codes in state A.

ZIPINC4 is missing (.) when the patient's ZIP code is missing, invalid, or outside of the United States.