Study Design and Datasets:
The Maryland Health Services Cost Review Commission (HSCRC) is an independent state agency that has a data repository containing administrative hospital data for the state of Maryland. The HSCRC includes deidentified patient-level data for all inpatient discharges and emergency department visits.
The Home Owners’ Loan Corporation (HOLC) was commissioned in the 1930s as part of the federal government’s efforts to provide housing and mortgage relief. Over 200 American cities had “residential security maps” drawn, detailing the level of risk of lending for mortgages in various neighborhoods within each city. These cities include places such as Chicago, Detroit, Philadelphia, San Francisco, New York, and Baltimore (1). Areas on the map were given a possibility of four different grades of A, B, C, and D, and were respectively designated by four colors, green, blue, yellow and red. The level of lending risk are as follows: green-colored areas (A) were “Best”, blue-colored areas (B) were “Still Desirable”, yellow-colored areas (C) were “Definitely Declining”, and red-colored areas (D) were as “Hazardous”.
Baltimore’s digitized HOLC map were obtained from the Mapping Inequality project by Richmond’s Digital Scholarship Lab. The map represents the 1930’s HOLC grades that demonstrate the degree of redlining in Baltimore and were read as a standard geospatial data interchange format that encodes geographic data structures.
Study Population:
We used the HSCRC database to identify all firearm injuries who presented to a hospital in Baltimore, Maryland from October 1, 2015 to December 30, 2020 using ICD-10 diagnosis codes for firearm injuries (W32, W33, W34, W72, X73, X74, X93, X94, X95, Y22, Y23, Y23, and Y35). This excludes firearm injuries that occurred but were not seen in a Maryland hospital setting (including deaths and/or minor injuries). As most of the redlined areas are in Baltimore, only patients with zip codes within the city limits were included in the study. Records which had missing zip codes, contained zip codes outside of Baltimore, and were not Maryland residents were excluded. A total of 36 Baltimore zip codes were used for our analysis.
Outcome measure:
The primary outcome is to examine whether living in Baltimore’s redlined areas is associated with higher rates of firearm injury. Our dependent variable is the count of healthcare encounters for firearm injuries that occur in each contemporary zip code of Baltimore and will be offset by the overall number of people that live in that zip code as provided by the U.S. Census. Using a negative binomial regression model, we examined the rate of healthcare encounters for firearm injuries in relation to historical HOLC grades. Chi square analyses were performed on demographic variables describing our study population, with a significance level p < 0.05. Our final adjusted regression model was based on running multiple univariable and multivariable models in a stepwise manner and evaluating each model’s AIC scores. All statistical analyses were performed using R statistical software (Version 4.2).
Redlining Identification and Geospatial Analysis:
Baltimore’s digitized HOLC maps were transformed into data frames using tidy and broom packages in R. After the creation of these data frames, they were plotted as colored polygon shapefiles based on geographic coordinates. Since this HOLC grade data is only available on a census tract level and since census tracts are more granular geographic entities than zip codes, we imputed the HOLC grade data to correspond with current Baltimore zip codes which allowed for an alignment with the zip code residence data provided by HSCRC. This spatial smoothing of census tract data to zip code level data allows us to examine commensurate HOLC associations by zip code.
The polygons represent the four graded HOLC designations and zip code boundaries. This HOLC map was then overlayed over another map containing Baltimore zip codes corresponding to zip code tabulation areas (ZCTAs) from the U.S. Census and the Mapping Inequality project. One issue commonly faced in similar studies is the difference in geographic units of analysis, as census tracts and zip codes do not have perfectly align with one another. However, given that ZCTAs are generalized areal representations of U.S. Postal Service zip codes, have very little differences from each other, and have been used as statistical entities by the U.S. Census, we chose to link ZCTAs and zip code data together.
To determine the boundaries and get a percentage value of the overlaps from the HOLC and zip code/ZCTA data, spatial analysis packages were used to estimate the overlap of the two polygon maps. HOLC IDs were linked to zip codes by area of land in meters squared. We then converted zip code groups to percentages using a columnwise sum and divided by the land areas to compute the columnwise average of HOLC coverage. HOLC letter grades were assigned quantitative units to allow for a measurement of redlining on a linear scale. The grade designations are as follows: A = 1, B = 2, C = 3, and D = 4. Areas with the higher redlining scores indicate higher HOLC grades of redlining.
Statistical Analysis:
Chi-square tests were used to evaluate differences in demographic characteristics. Univariable comparisons between redlined and non-redlined zip codes were conducted, followed by a negative binomial regression model to assess the association between HOLC grades and firearm injury. Final model selection was based on AIC scores. All analysis were performed using R statistical software (Version 4.2)
Median income, education, and ethnicity were excluded from the adjusted negative binomial regression model due to its high collinearity with our outcome variable of firearm injury. The decision to exclude these variables was made due to multicollinearity issues. Excluding these variables further ensured the validity of this study’s model results. These exclusions may limit our interpretations of our findings as income, education, and ethnicity have been identified as important predictors of firearm injuries. Lastly our analysis is cross-sectional, meaning that we cannot establish causality and can only infer that an association exists between redlining and firearm injuries.
Several negative binomial regression models were examined on a univariate and multivariate level. Population size was controlled in all models and included as an offset parameter for our models. Based on the study population demographics and the current body of literature on redlining in Baltimore, our covariate of interest was median age (6, 8, 10-12). Once univariate models were performed by zip code with their corresponding redlining scores, the coefficients, intercepts, and AIC score were compared with one another.