As determined by the Health Resources and Services Administration based on UDS data, the ratio of patients (from health centers with 11 or more patients in that ZCTA) to a matching sub-population (such as the total population or low-income population). For example, for each ZCTA the number of reported health center patients is divided by the number of low-income or total residents. Note: Because the UDS does not distinguish the income of health center patients at the ZIP Code level, this measure is not perfect, particularly when dividing the number of health center patients by the number of low-income (as not all health center patients are low-income below 200% FPL). But because it is known that approximately 89.65% of health center patients and 86.88% of look-alike patients nationally are low-income, the basic utility of the calculation in assessing the role of health centers in serving the community is not changed. Such penetration rate measures should be considered the starting point for exploring potential need, not the final answer.