Re: Soares et al.: Patient accessibility to eye care in the United States (Ophthalmology. 2023;130:354–360)
Authors: Diane M. Gibson
Journal: Ophthalmology (2023)

Soares et al1Soares R. Mokhasi N. Sharpe J. et al.Patient accessibility to eye care in the United States.Ophthalmology. 2023; 130: 354-360Abstract Full Text Full Text PDF PubMed Scopus (3) Google Scholar state that the purpose of their study is to "(1) determine the applicability of primary care health provider shortage area (HPSA) designations to eye care providers (ECPs) and (2) to propose a new model using 2-step floating catchment areas to define access to ECPs more accurately." This letter argues that the methods Soares et al1Soares R. Mokhasi N. Sharpe J. et al.Patient accessibility to eye care in the United States.Ophthalmology. 2023; 130: 354-360Abstract Full Text Full Text PDF PubMed Scopus (3) Google Scholar used for their first purpose are not valid for their data and that the paper does not present evidence that access to ECPs is measured more accurately using their approach than using other approaches. Soares et al1Soares R. Mokhasi N. Sharpe J. et al.Patient accessibility to eye care in the United States.Ophthalmology. 2023; 130: 354-360Abstract Full Text Full Text PDF PubMed Scopus (3) Google Scholar argue that "our work demonstrated the insufficiency of federal HPSA designation to characterize areas of shortage and inaccessibility to ECPs adequately" on the basis that "the current HPSA designations correlate only weakly with ECP density or areas of high visual impairment." In their correlation analysis, Soares et al1Soares R. Mokhasi N. Sharpe J. et al.Patient accessibility to eye care in the United States.Ophthalmology. 2023; 130: 354-360Abstract Full Text Full Text PDF PubMed Scopus (3) Google Scholar use census tracts as the unit of analysis and each tract-level observation has a tract-level HPSA score and county-level measures of ECP density, vision loss, and blindness. However, Spearman correlation analysis is not appropriate for these data. For the standard test of the statistical significance of a Spearman correlation coefficient to be valid, it is necessary that "each pair of x-y values is measured independently from each other pair."2Shober P. Boer C. Schwarte L. Correlation coefficients: appropriate use and interpretation.Anesth Analg. 2018; 126: 1763-1768Crossref PubMed Scopus (4124) Google Scholar This requirement is not met given that each tract-level observation within a county has the same values for provider density, vision loss, and blindness (in other words, there is spatial correlation between tracts in a county). Furthermore, given that there is no variation between tracts in a county in the values of the county-level variables, tract-level within-county correlation between tract-level and county-level variables is undefined.2Shober P. Boer C. Schwarte L. Correlation coefficients: appropriate use and interpretation.Anesth Analg. 2018; 126: 1763-1768Crossref PubMed Scopus (4124) Google Scholar County-level variables should only be used in the calculation of county-level correlations. Similarly, there are problems with the analysis of whether the number of tracts designated as HPSAs differ with whether or not a tract is in the lowest quartile of ECP density per county and with whether or not a tract is in the highest quartile of county-level vision loss, as observations must be independent for χ2 tests to be valid.3Nowacki A. Chi-square and Fisher's exact tests.Cleve Clin J Med. 2017; 84: e20-e25Crossref PubMed Google Scholar Although areas with HPSA designation may in fact be poor proxies for areas with limited access to ECPs, the analysis presented by Soares et al1Soares R. Mokhasi N. Sharpe J. et al.Patient accessibility to eye care in the United States.Ophthalmology. 2023; 130: 354-360Abstract Full Text Full Text PDF PubMed Scopus (3) Google Scholar does not make this case convincingly. Soares et al1Soares R. Mokhasi N. Sharpe J. et al.Patient accessibility to eye care in the United States.Ophthalmology. 2023; 130: 354-360Abstract Full Text Full Text PDF PubMed Scopus (3) Google Scholar define a county-level patient accessibility score (PAS) as equal to the county-level poverty rate multiplied by the county-level prevalence of visual impairment divided by an index representing patient access to ECPs from the geographic center of a county calculated using the 2-step floating catchment area approach with 60-minute driving time catchment areas. A higher PAS score is interpreted as worse accessibility. The typical 2-step floating catchment area approach measures patient access to providers from multiple locations within units of geography such as counties.4Luo W. Wang F. Measures of spatial accessibility to health care in a GIS environment: synthesis and a case study in the Chicago region.Environ Plann B Plann Des. 2003; 30: 865-884Crossref PubMed Scopus (1123) Google Scholar Assessing ECP accessibility from only one location in a county as in Soares et al1Soares R. Mokhasi N. Sharpe J. et al.Patient accessibility to eye care in the United States.Ophthalmology. 2023; 130: 354-360Abstract Full Text Full Text PDF PubMed Scopus (3) Google Scholar raises the concern that the measure may not be a good proxy for ECP accessibility for individuals who live elsewhere in the county. This concern is exacerbated because US counties can be very large, and the geographic center of a county may be a large distance from where the bulk of the population in a county is located.5Thomas Dilts. Mapping with population-weighted centroids.www.esri.com/about/newsroom/arcuser/popcentroids/Date accessed: July 6, 2023Google Scholar Soares et al1Soares R. Mokhasi N. Sharpe J. et al.Patient accessibility to eye care in the United States.Ophthalmology. 2023; 130: 354-360Abstract Full Text Full Text PDF PubMed Scopus (3) Google Scholar argue, "In our study, the fact that lack of insurance has a significant association with higher PAS validates our scoring system as a reasonable measure of the many facets of accessibility. Our more nuanced analysis may be more accurate than previous standard density-based availability analysis." However, this finding does not speak to whether the PAS is more accurate than density-based measures of ECP availability. Given that there is no measure of actual ECP accessibility to compare to alternate measures of ECP accessibility, the only way to assess whether a measure is better is to choose an outcome theoretically related to the accessibility of ECPs, such as the use of eye care services, and then determine whether PAS or ECP density-based measures of ECP accessibility have superior predictive validity for this outcome. Patient Accessibility to Eye Care in the United StatesOphthalmologyVol. 130Issue 4PreviewThe United States (US) federal government uses health provider shortage areas (HPSAs) to define patient accessibility to primary care physicians. It is unclear whether HPSAs can be applied to eye care providers (ECPs). Our study determined the applicability of federal HPSA designations to ECP availability in the US. Full-Text PDF ReplyOphthalmologyVol. 131Issue 3PreviewWe would like to take this opportunity to respond to Dr Gibson's commentary about our article and thank them for critically analyzing our work. Full-Text PDF