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Constructing an Index of Food Insecurity
As part of a project for Forgotten Harvest, a Detroit area food bank, Data Driven Detroit was asked to construct a single measure that would quantify food insecurity levels by census tract in the tri-county area of southeast Michigan (Wayne, Oakland, and Macomb Counties). The measure, dubbed the Food Insecurity Index, had to satisfy four criteria. It had to
- be based on publicly available data at the census tract level;
- incorporate multiple factors affecting food insecurity;
- be independent of direct measures of food insecurity such as interview data with residents; and
- be methodologically sound but readily understood.
Defining the indicators in the index
Household finances are recognized as central to a household’s ability to provide food for its members 1. Accordingly, household finances became the measure around which we selected other variables to broaden the index. As a first step, then, we constructed four indicators at the census tract level of household finances. These four were:
- median household income;
- count and percentage of households receiving Supplemental Nutrition SNAP benefits;
- count and percentage of households below the poverty level; and
- count and percentage of households with housing costs greater than 30% of household income, which we labeled “housing burden.”
In order to select one of the four measures of household finances we examined each measure’s relationships with other potential measures of food insecurity such as lack of a personal vehicle. Our goal was to select the household finances indicator that was related to the other potential indicators but not so strongly related that the index would essentially consist of multiple measures of the same concept. The housing burden variable best fit these criteria. Ultimately, the Food Insecurity Index was composed of the housing burden measure and the following five measures, four of which were drawn from the 2010-2014 Five-Year American Community Survey:
- households with no vehicle – the count or percentage of households in a tract with no vehicle available
- individuals with less than a high school diploma – the count or percentage of individuals age 25 years or older in a tract with less than a high school diploma
- low-income movers — the percentage or count of individuals in a tract with incomes less than $25,000 who moved in the past year
- beyond 1/4 mile of transit– the percentage of land area in a tract more than one-quarter mile from a bus stop
- single-parent households – the percentage of families in a tract with children younger than 18 years of age that are headed by a single parent
Method of constructing the index
To construct the index we utilized a slightly modified version of the methodology used by the Census Bureau in the mid-2000s to identify census tracts predicted to have low response rates in the 2010 census 2. For the Food Insecurity Index, each one of the six indicators was sorted from low to high across the 1100+census tracts. For each indicator, each of the tracts was assigned a number from 1 to 11, with a “1” indicating a low likelihood of low food insecurity and “11” a high likelihood of food insecurity. Finally, for each census tract, the scores were summed across the six variables to get the Index value, with values ranging from 6 to 66.
The final step in constructing the Index was deciding whether to represent all six indicators as percentages or to rely on numbers for four of the indicators and percentages for the ¼ mile from transit and single-parent household indictors. We constructed the Index both ways and compared the results. While the two indexes painted similar pictures of the geography of food insecurity in the tri-county area, we decided that the index that included numbers for the first four variables rather than percentages gave a clearer picture of the magnitude of the problem.
Scores on the Index
Figure 1 illustrates the distribution of scores of the Index for the three counties. The lowest score for any census tract was 8 (indicative of low food insecurity), while the highest score was 65. The median score was 38. The distribution looks roughly bell shaped.
An advantage of the methodology used to construct the Index is that it recognizes and incorporates variation within census tracts. For example, there are large portions of Wayne, Oakland, and Macomb Counties that are not served by buses. Census tracts in those areas are therefore likely to have a relatively high score on the “beyond quarter mile of transit” indicator even though they may have low scores on the other measures that would otherwise indicate food insecurity. Due to this their overall Food Insecurity Index score would be in the lower range of the Index despite their lack of proximity to a bus stop.
Figures 2 through 4 show the distribution of Food Insecurity Index scores for each of the three counties. In comparing the three county graphs, the salient feature is the shape of the distributions, not the total number of tracts per Index value, since the three counties have different numbers of tracts.
The median Index score for Macomb County census tracts was 40, with scores skewed to the higher values.
Oakland County, in contrast, had a median score of 33. Its distribution is skewed toward lower scores, indicating low food insecurity. Nonetheless, there were numerous tracts with high scores.
Wayne County’s median score of 39 masks the wide variation in scores across the county, denoting quite a few tracts that are less likely to face food insecurity but far more where it was a problem.
Map of Food Index Insecurity Index Values in Wayne, Oakland, and Macomb Counties
While Figures 1 through 4 illustrate the variety of levels of food insecurity in the three counties of southeast Michigan, they do not locate the differences geographically. That is displayed in Figure 5.
Overall, these populations are heavily concentrated around Detroit and Pontiac, often in areas that are fairly well served by existing and potential Forgotten Harvest (FH) and Gleaners Community Food Bank (GCFB) agencies. However, the Food Insecurity Index also identified several other areas of potentially high need, including several that are not currently well-covered by the existing FH/GCFB network. These areas include:
- Madison Heights, between I-75 and the Oakland County Line
- Wixom, between Pontiac Trail and I-96
- Romulus, east and south of Detroit Metropolitan Airport
- Roseville and Clinton Township, along Gratiot Avenue
- Southfield, near the I-696/US-24 interchange
- Van Buren and Huron Townships in southwestern Wayne County
- Portions of Chesterfield Township and New Baltimore in Macomb County
Of the Top 20% of tracts that were most likely to be food-insecure (having Index scores of 49-65), 63 of the 211 were located in Detroit. This represented over 20% of the city’s census tracts. However, 148 of the 852 census tracts in outer suburbs also placed into this range, demonstrating that food insecurity is a regional problem.
For example, Novi and Wixom in southwestern Oakland County are often considered affluent areas with few residents in need of assistance, but the Index data showed that there is actually quite a bit of need that may not be being met. Those two tracts had index scores of 60 and 54, respectively, out of a potential 66.
The Food Insecurity Index was constructed for two main purposes: First, Forgotten Harvest wanted to identify food insecure areas in Wayne, Oakland, and Macomb Counties that are not served by either Forgotten Harvest or Gleaners. Second, Forgotten Harvest wanted to know whether there were areas currently served by FH or GCFB that are food secure enough that FH resources could more profitably be deployed elsewhere.
The Food Insecurity Index addresses these purposes in multiple ways. By analyzing the data by census tract, the Index can isolate levels of food insecurity at a lower level of geography than has been done before and adjust the service areas to better meet the need.
Moreover, the Index, composed of publicly available data measures, recognizes that food insecurity is multidimensional. It incorporates census tract level measures of households’ financial ability to buy food and the amount of financial risk they have, indicated by the housing burden measure; their access to means of traveling to places where they can obtain food; their residential stability (for low-income households); their level of education; and an aspect of their household composition (percentage of single parent families).
In addition, by its multidimensional nature, the Index recognizes that a census tract is not necessarily a homogeneous entity. For example, while some tracts may be uniformly well off or poorly off on all the measures included in the Index, many are mixtures. Examining the scores on each of the six variables for high risk food insecurity census tracts can help identify the factors associated with the insecurity.
The Food Insecurity Index numbers are just the starting points for understanding food insecurity in an area. Beyond examining the scores on the individual components of the Index, the analyst can bring in other information such as the unemployment rate; use of SNAP benefits; average family size; ethnic composition; and availability of full service grocery stores. And it would benefit understanding to look for common factors in contiguous tracts with similar scores.
Finally, Index scores can be used as conversation starters with people in food insecure areas. Such conversations frequently bring to light factors that researchers and organizations have not known about or considered but that may be important contributors to food insecurity.
1 See for example. “Household Food Security in the United States in 2015, ERR-215. Economic Research Service/USDA.
2 Antonio Bruce and J. Gregory Robinson, “Tract Level Planning Database with Census 2000 Data.” U.S. Census Bureau March 2007.