BORDER CROSSING FOR HOSPITAL-CARE AND ITS IMPLICATIONS FOR THE USE OFSTATEWIDE DATA

A major concern of researchers using state data sets for population-ba sed analyses and market share studies in the health care sector is the potential bias caused by 'border crossing'-patients receiving care ou t of state. By using the Health Care Financing Administration (HCFA) d ischarge abstract files for 1987 and 1988, we found that 'border cross ing' is not a serious problem for the two large states we examined. On ly 4.4% of New York patients and 2.15% of California patients received care out of state. At the county and zip code level, 'border crossing ' is more frequent but tends to be concentrated in areas adjacent to o ther states. Even excluding all zips with more than 10% of patients cr ossing the 'border' results in a small loss of patients (2.2% for New York and 1.0% for California).