Assessing specification assumptions in urban pharmacy accessibility in New York City

Accurate measurement of pharmacy access is central to understanding spatial inequities in healthcare availability. The two-step floating catchment area (2SFCA) method is widely used to integrate proximity and availability, yet key challenges remain unresolved in dense urban settings, including catchment sensitivity, and the lack of uncertainty quantification despite reliance on survey-based population estimates.

To assess catchment sensitivity to specification using a mode-adjusted 2SFCA approach while propagating uncertainty from the American Community Survey (ACS).

We conducted a cross-sectional analysis of pharmacy access across 2222 census tracts in New York City using multiple distance- and time-based catchments. We applied a mode-adjusted 2SFCA method that incorporates walking and driving accessibility by decomposing populations demand using tract-level transportation proportions from the ACS. Uncertainty in access estimates was quantified via hierarchical Monte Carlo simulation propagating the margins of errors from the ACS. We used cross-tabulation to quantify agreement in classifying tracts as having fewer than one pharmacy per 10,000 residents across transportation modes.

Relative pharmacy access patterns were stable across a wide range of catchment specifications, indicating limited sensitivity to reasonable changes in distance or travel-time thresholds in this dense urban setting. The mode-adjusted approach increased median access and introduced additional variability by accounting for transportation differences. Uncertainty in access estimates was generally negligible but may matter where driving and walking catchments do not overlap.

These findings suggest that, in New York City, catchment size and ACS uncertainty have limited impact on access estimates, while transportation differences and border effects play a larger role.