README.md

In aghaynes/HSAr: Delineate Hospital Service Areas from spatial shapefiles and flow data

output: github_document bibliography: references.bib

HSAr - Automated HSA delineation through R

Introduction

Hospital service areas (HSAs) are often used in healthcare analyses, particularly for studying variation in rates, cost and quality. Rather than being tied to geopolitical regions (such as counties, states or cantons), HSAs are defined using patient movement and thus reflect patients usage patterns.

Originally described in the 1970's (Wennberg & Gittelsohn 1973), the classical approach to defining HSAs has changed little since then and involves some highly subjective decisions, and the method is thus not reproducible. HSAr is an attempt to create a reproducible method to create HSAs using the open access R language (as such it's use is not hampered by pay walls and only a modest knowledge of R should suffice to use it).

This method is described in a paper provisionally accepted at Health Services Research.

Installation

R itself

R is available from CRAN. I also recommend RStudio, which provides alot of functionality to aid working with R.

Installing HSAr from github with devtools

devtools::install_github("aghaynes/HSAr")

Using HSAr

Load the package

The package is loaded in the normal manner for R packages. I also recommend setting stringsAsFactors to FALSE as this frequently causes unexpected behaviour.

library(HSAr)
options(stringsAsFactors = FALSE)

Load your datafiles

Two types of data are required for HSAr

1. a shapefile defining the spatial relationship between regions
2. source and destination of individuals

Spatial data

There are many ways to get shapefiles into R. A common one is using the rgdal package which can read OGR datafiles (e.g. ESRI shapefiles).

library(rgdal)
file <- system.file("extdata", "shape.shp", package = "HSAr")
shape <- readOGR(dirname(file), "shape", stringsAsFactors = FALSE)
# the map can be viewed with the plot function
plot(shape) 

Whichever way you choose to use to get your spatial data into R, it needs to be a SpatialPolygonsDataFrame object (as opposed to a sf object). If you have an sf type object, as(x, "Spatial") can be used.

HSAr has an example shapefile which can be loaded via

data(shape)
plot(shape)

The rownames of the spatial data should be consistent with the names in the patient data. It is normally necessary to do this after importing the shapefile from an ESRI file.

shape <- spChFIDs(shape, shape$reg) # not necessary in this case, but this is how you could do it

Patient/flow data

There are also numerous ways to get tabular data into R. See e.g. read.csv (from the utils package), the functions in the readr (similar file types to the utils package), readxl, openxlsx (for excel data) and/or haven (for datasets in SPSS, Stata or SAS formats) packages.

flowdata <- read.csv("path_to_data.csv")
# have a look at the contents
# head(flowdata)

A demo dataset is included in the package, containing the home and hospital locations of 1178 patients what live in the region contained in the shape shapefile above.

data(flow)

head(flow)

##   from to
## 1    I  J
## 2    I  O
## 3    I  J
## 4    I  O
## 5    I  S
## 6    I  S

table(flow$from) # where they live

## 
##   A   B   C   D   E   G   H   I   J   K   L   M   O   Q   R   S   T   U 
##  40 100  66  67  82  44  67  42  34  56  59  20  48  98  67  78  89  91 
##   V 
##  30

table(flow$to) # where they received care

## 
##   C   J   O   S 
## 318 321 184 355

There are patients travelling from 19 to 4 regions. Note that there are 21 regions in the shapefile, so no one from 2 regions were hospitalized.

It is possible to summarize the patient flows using the flows function, which provides details of the number of patients moving between regions and the proportions of the regions that the given number of patients represents.

f <- flows(flow$from, flow$to)
head(f)

##   from to  N N_from N_to prop_from     prop_to rank_from rank_to
## 1    A  C 26     40  318      0.65 0.081761006         1       5
## 2    A  J 10     40  321      0.25 0.031152648         2      15
## 3    A  O  2     40  184      0.05 0.010869565         3      19
## 4    A  S  2     40  355      0.05 0.005633803         4      18
## 5    B  C 55    100  318      0.55 0.172955975         1       1
## 6    B  J 27    100  321      0.27 0.084112150         2       2

We see that 26 people move from A to C (65% of those in A and 8% of those that go to C). Ranks are also provided (most from A go to C - rank is 1 - and A represents the 5th most people going to C).

Exploring maps

HSAr also provides a functions for exploring maps. You might want to look at a specific HSA for example. The minimap function allows you to zoom into a given HSA (or more generally, a polygon in a shapefile) by providing suitable a regular expression to identify it (see ?regex). The selected regions are highlighted in green.

# Show all regions
minimap(shape)

# Show region B
minimap(shape, polygon = "B")

# Show region B or C
par(mfrow = c(1,3))
minimap(shape, polygon = "B|C")
# equivalently, minimap(shape, polygon = "[BC]")
# changing the zoom is also possible, which is particularly useful for larger maps
minimap(shape, polygon = "B|C", zoomout = .01)
minimap(shape, polygon = "B|C", zoomout = .5)

For our example, we could look at which regions have hospitals (or at least receive patients; left), and which receive no patients (right):

par(mfrow = c(1,2))
minimap(shape, polygon = paste(unique(flow$to), collapse = "|"), zoomout = 0)
minimap(shape, polygon = paste(shape$reg[!shape$reg %in% flow$from], collapse = "|"), zoomout = .75)

Generating HSAs

The goal of HSAr is to make the generation of HSAs quick and easy. The gen_hsa (generate HSAs) function is the main work-horse of the package. It iteratively looks at each region which receives patients, identifies the neighbours and merges them with the hospital region if most flow is in that direction. It will keep doing that until it has allocated all regions.

Specific details (click to expand)

As an example, if we look at region S, the algorithm identifies the neighbours (H, I, P, T, V), then finds looks at from which most patients go to the focal hospital (H, I, T, V) and lumps those regions together with the hospital for the next iteration (remember that P has no patients, so that region doesn't appear in the flows, which are basically just cross tabulations).

##    from to  N rank_from
## 25    H  S 33         1
## 29    I  S 18         1
## 65    T  S 65         1
## 73    V  S 17         1

The next iteration for hospital S then looks at regions neighbouring S+H+I+V+T and merges them if most patients go to S (or another hospital within S+H+I+V+T, if that should be the case), which might be region U. In this way, HSAs grow organically around the hospitals.

Once no more changes are necessary, the algorithm will check that each "proto-HSA" is valid - that it meets the requirements of having a high enough localization index (proportion of people remaining in the HSA, defaults to 0.5 = 50%) and number of interventions (defaults to 10). At this point, regions without patients would fail both of these tests and the algorithm estimates the regions flow based on the flows of the neighbours. In this case, it is allocated to S.

##   to   N
## 4  S 191
## 2  J 113
## 3  O  67
## 1  C  39

Proto-HSAs that do have patients, but fail either due to low LI or number of intervention are merged with the proto-HSA receiving most flow.

Once this has been done, the algorithm starts again to ensure that no more HSAs should be merged and all HSAs are valid.

By default, gen_hsa prints the number of HSAs at each iteration (as it can take some time to run) and will plot a map of the current allocations at the beginning of each iteration. The number of iterations required depends the number of regions. It will also warn about missing regions. For instance, we see that two regions in the shape file do not exist as sources, and 17 do not exist as destinations.

par(mfrow = c(2,4))
hsas <- gen_hsa(shp = shape, from = flow$from, to = flow$to)
## Warning: 2 regions in 'shp' not in 'from'. 
## Warning: 17 regions in 'shp' not in 'to'.
## Iterating
## ....
##  11 
## ....
##  7 
## ...
##  6 
## ...
##  3 
## ...
##  3
## 
## HSAs produced 3

Summary, plot and minimap methods exist for the returned hsa object, making it easy to view the results.

par(mfrow = c(1,3))
summary(hsas, plot = TRUE)

## Criteria
##    Localization Index (LI) threshold:    0.4 
##    Minimum interventions:                10 
## Iterations required to satisfy criteria: 5 
## 
## Shapefile
## Number of regions in original shapefile: 21
## 'shp' regions not in 'from' (2): 
## N P
## 'shp' regions not in 'to' (17): 
## N P
## 'from' regions not in 'shp' (0): 
## 
## 'to' regions not in 'shp' (0): 
## 
## HSAs generated (3):
## C J S
## Regions assigned to HSAs via:
##         Var1 Freq
##  destination    3
##         flow   15
##    neighbour    3
## 
## 
## Localization Index
##    Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
##  0.5600  0.5770  0.5940  0.5950  0.6125  0.6310 
## 
## Interventions
##    Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
##   318.0   336.5   355.0   392.7   430.0   505.0 
## 
## Creating 3 plots

Most important here are the localization index and number of interventions sections at the end. The plot option can be used to show three figures depicting the number of regions at each iteration of the loop and the number of interventions and localization index in the resulting HSAs. Options li and n_interv will also return these numbers in a table.

Both plot and minimap methods show the regions making up the HSAs as well as the HSAs themselves (in grey), but minimap adds labels to the map.

par(mfrow = c(1,2))
plot(hsas)
minimap(hsas)

The final shapefile and a lookup table to assign regions to HSAs are accessed via

hsa_shape <- hsas$shp
hsa_lookup <- hsas$lookup

These files can then be saved for later use in R or other software.

A clustering based approach

A clustering based approach to creating hospital regions was published by Delamater et al (2013), together with code for its implementation. A modified version is provided here, as it is discussed in the provisionally accepted paper. Where original method used hospital beds and staffing to define the similarity among hospitals, we use the distance between regions and the patient flows. The method uses a two-step K-means and Wards clustering approach to first derive the best number of clusters (HSAs) and then assign regions to a cluster. Compared to the more classical approach, this method can produce very different results and might be better for cases where there are already more distinct regions in the data (e.g. fewer, larger hospitals which have natural catchment areas, rather than cases where there are many hospitals with largely overlapping catchments). The example data provided in the package, for example, was designed to produce three HSAs, which it does using the method above. Using the cluster-based approach it produces only two HSAs.

Besides using the distance between the regions to define the similarity, we also made two other changes to the original implementation:

1. The original method for identifying the number of clusters does not perform well when there are few hospitals. A second approach is provided through the peak argument which can take values of dela (short for delamater) or diff (which is the R function used in the implementation, short for difference). If the default setting produces an error, try the other setting, as in the example below.
2. As with the HSAr method above, the cluster based approach cannot allocate regions without flow to a HSA. As above, we have implemented the same filling algorithm here. To turn it off, set the fill option to FALSE.

chsas <- gen_hsa_clust(shp = shape, from = flow$from, to = flow$to, max_clusters = 21, peaks = "diff")

## Warning: 2 regions in 'shp' not in 'from'.

## 
## Filling N with 236 observations
## Filling P with 646 observations

## Warning: 17 regions in 'shp' not in 'to'.

## ....

par(mfrow = c(1,2))
plot(chsas$shp)
plot(hsas)

References

Delamater, Paul L., Ashton M. Shortridge, and Joseph P. Messina. 2013. “Regional Health Care Planning: A Methodology to Cluster Facilities Using Community Utilization Patterns.” *BMC Health Services Research* 13 (1):333. .

Wennberg, John, and Alan Gittelsohn. 1973. “Small Area Variations in Health Care Delivery.” *Science* 182. .

aghaynes/HSAr documentation built on Sept. 20, 2021, 5:29 p.m.