3.1 Descriptive statistics
Compared with 1993, in 2011, the percents of sit-down restaurants and supermarkets in the study area increased 10.1 and 3.3 percentage points, respectively (Table 1). Our study area’s population in 2011 (compared with 1993) tended to be older (45-64 or 65 or above), more non-white, more college educated or higher, and having higher household incomes. The study area had a greater population density, greater mix of land use, and greater percent of single-family housing in 2011 compared with 1993.
Table 1 Selected characteristics of neighborhoods in years 1993, 2001 and 2011, Twin Cities Region
Neighborhood characteristic
|
1993 a
|
2001
|
2011
|
Change b
|
P value c
|
Number of observations (neighborhoods)
|
2,083
|
2,083
|
2,083
|
---
|
---
|
|
|
|
|
|
|
Relative availability of sit-down restaurants and supermarkets
|
|
|
|
|
|
Percent of sit-down restaurants d, mean (SD)
|
16.1 ± 33.1
|
22.7 ± 36.4
|
26.2 ± 36.8
|
10.1 ± 41.4
|
<0.05
|
Percent of supermarkets e, mean (SD)
|
2.0 ± 11.9
|
2.4 ± 12.9
|
5.3 ± 19.6
|
3.3 ± 20.1
|
<0.05
|
|
|
|
|
|
|
Built environment characteristics
|
|
|
|
|
|
Residential population density, 1,000 person/km2,
median (IQR)
|
1.2 (1.6)
|
1.2 (1.3)
|
1.3 (1.3)
|
0.0 (0.5)
|
<0.05
|
Employment population density, 1,000 person/km2,
median (IQR)
|
0.6 (0.8)
|
0.7 (0.7)
|
0.7 (0.7)
|
0.0 (0.1)
|
<0.05
|
Mix of land use f, median (IQR)
|
45.0 (48.0)
|
51.3 (49.7)
|
57.0 (46.0)
|
8.7 (20.4)
|
<0.05
|
Percent of single-family housing g, median (IQR)
|
68.5 (65.4)
|
73.2 (64,9)
|
94.9 (36.8)
|
16.3 (27.0)
|
<0.05
|
Total sit-down restaurants and fast food restaurants,
median (IQR)
|
0.0 (1.0)
|
0.0 (2.0)
|
1.0 (2.0)
|
1.3 (3.2)
|
<0.05
|
Total supermarkets, grocery stores and convenience
stores, median (IQR)
|
0.0 (1.0)
|
1.0 (1.0)
|
1.0 (2.0)
|
0.2 (1.2)
|
<0.05
|
|
|
|
|
|
|
Sociodemographic characteristics
|
|
|
|
|
|
Age, mean (SD)
|
|
|
|
|
|
Percent of population under 14
|
22.2 ± 6.4
|
21.3 ± 6.8
|
19.9 ± 6.0
|
-2.3 ± 4.8
|
<0.05
|
Percent of population 15−29
|
23.7 ± 7.3
|
20.9 ± 7.8
|
21.6 ± 8.4
|
-2.1 ± 4.6
|
<0.05
|
Percent of population 30−44
|
27.2 ± 4.7
|
26.0 ± 4.8
|
21.9 ± 4.5
|
-5.3 ± 5.0
|
<0.05
|
Percent of population 45−64
|
16.9 ± 5.0
|
21.2 ± 5.4
|
25.8 ± 5.7
|
8.9 ± 5.8
|
<0.05
|
Percent of population 65 or above
|
9.9 ± 6.6
|
10.5 ± 7.3
|
10.8 ± 6.0
|
0.9 ± 5.7
|
<0.05
|
Percent of population with education level of college or
above, mean (SD)
|
57.6 ± 15.2
|
66.1 ± 14.9
|
68.1 ± 14.3
|
10.5 ± 8.6
|
<0.05
|
Race, median (IQR)
|
|
|
|
|
|
Percent of white race
|
96.0 (5.0)
|
90.6 (13.0)
|
87.0 (16.0)
|
-10.8 (11.6)
|
<0.05
|
Percent of black race
|
1.0 (3.0)
|
2.9 (5.1)
|
4.0 (9.0)
|
4.1 (7.2)
|
<0.05
|
Median household income h, $1,000, mean (SD)
|
38.2 ± 12.5
|
40.5 ± 15.1
|
37.1 ± 14.5
|
0.4 ± 7.5
|
<0.05
|
Time elapsed from 1993, year, mean (SD)
|
0 ± 0
|
8 ± 0
|
18 ± 0
|
18 ± 0
|
---
|
IQR, interquartile range; SD, standard deviation.
a Because we did not have neighborhood built environment and sociodemographic data for 1993, we assumed that data for 1990 would be valid substitutes for the missing 1993 data
b Change in neighborhood characteristics from year 1993 to 2011.
c P value for one-tailed Student’s t-test of difference in means and Kruskal-Wallis H test of difference in medians from years 1993 and 2011
d Percent of sit-down restaurants relative to total sit-down restaurants and fast food restaurants.
e Percent of supermarkets relative to total supermarkets, grocery stores and convenience stores.
f The mix of land use was measured by 3-tier land use entropy (denominator set to the static 3 land use type in the census block group), which used three land use categories (residential, employment and retail) to calculate mix of land use in the block group.
g Percent of single-family housing relative to total single-family and multi-family housings.
h The median household incomes in 1993 and 2001 were adjusted for inflation to compare with that in 2011.
3.2 Results from cluster analyses: neighborhood type (Year 1993)
The six robust neighborhood types that we defined by the final cluster solution represented non-overlapping groupings of Twin Cities Region neighborhoods based on built environment and sociodemographic attributes in 1990 (the first observational year). These clusters included: cluster 1 - high-density urban core; cluster 2 - low-income, non-white inner city; cluster 3 - urban; cluster 4 - aging suburb; cluster 5 - high-income suburb; and cluster 6 - suburban edge.
We labeled clusters based on their most prominent built environment and sociodemographic characteristics (See Table S2 in the Additional File 1). Compared with most of the other clusters, cluster 1, “high-density urban core”, had relatively greater levels of residential and employment population densities, a greater mix of land use, comparatively lower percent single-family housing, comparatively higher percent population aged 15-29, and comparatively lower percent population aged under 14. Cluster 2, “low-income, non-white inner city”, had moderate-to-high residential and employment population densities and comparatively higher percent non-white race population, relatively lower level of median household income and comparatively lower percent population with a college education or above. Cluster 5 and Cluster 6, “high-income suburb” and “suburban edge”, had relatively lower levels of residential and employment population densities, lower degrees of mix of land use, and relatively greater levels of median household income. Cluster 3 (“urban”) and Cluster 4 (“aging suburb”) had moderate levels of almost all neighborhood features, except for a greater degree of mix of land use and comparatively higher percent population aged 65 and above.
Figure 1 shows that the high-density urban core (abbreviated as urban core) and low-income, non-white inner city (abbreviated as inner city) neighborhoods were tightly clustered in a small segment within the municipal boundaries of the Twin Cities. Urban and aging suburb neighborhoods comprised those transitional areas located between the urban core or inner city neighborhoods and the suburban areas. Another small grouping of aging suburb and high-income extended into the counties of Carver and Scott and the county of Washington, respectively. The generated clusters reflected comprehensive but distinguishable physical and sociodemographic environments.
3.3 Relationship between neighborhood type and relative availability of sit-down restaurants and supermarkets”
Table 2 shows the post-estimated linear contrasts of percent of sit-down restaurants and percent of supermarkets in the neighborhood by year and for each neighborhood type pair from the multivariable linear mixed effects regression models. For example, the coefficient of 23.02 in 1993 in the sit-down restaurant model (Table 2) suggested that the percent of sit-down restaurants in urban core was 23.02 percent higher than that of inner city in 1993. And the confidence interval of 13.18-32.85 suggested that we had 95% confidence that the real difference in the percent of sit-down restaurants between urban core and inner city fell between 13.18 and 32.85 in 1993. Urban core neighborhoods had a higher percent of sit-down restaurants (by 22.78-27.95 percentage points) compared with the other five types of neighborhoods in 1993 (Table 2); we did not observe any differences in the percent of supermarkets for 1993 (Table 2). For 2001, we observed more differences in percent of sit-down restaurants and supermarkets by neighborhood type (Table 3). Inner city neighborhoods had a higher percent of sit-down restaurants (by 8.19 percentage points) than did suburban edge neighborhoods; aging suburb neighborhoods had slightly more supermarkets (1.59-1.78 percentage points) compared with the urban, and suburban edge neighborhoods. In 2011, inner city neighborhoods had more sit-down restaurants (8.57-12.27 percentage points) than the urban, aging suburb, and high-income suburb neighborhoods (Table 4). Figures 2 and 3 show the estimated mean of percent of sit-down restaurants and supermarkets over time for each neighborhood type from the same models. Although urban core neighborhoods had a consistently greater percent of sit-down restaurants compared with other neighborhoods in all (three) observational years (Figure 2), the differences between urban core and the other three types of neighborhoods (inner city, high-income suburb, and suburban edge) decreased in 2011 compared with 1993 and 2001. Table S5 in Additional File 1 shows the p values for the changes of differences in estimated mean of percent of sit-down restaurants/supermarkets for each neighborhood type pair between two observation years from the same models. The results in Figures 2-3 and Tables S6-S7 in the Additional File 1 were derived from the same models as for Tables 2-4. We performed correlation analysis and did not find high correlation between covariates in the mixed effects models.
Table 2 Contrastsa of percent of sit-down restaurantsb and percent of supermarketsc for each neighborhood typed pair in 1993
Sit-down restaurant model: Estimated beta (95% confidence interval)
|
|
Urban core
|
Inner city
|
Urban
|
Aging suburb
|
High-income suburb
|
Suburban edge
|
Urban core
|
--
|
--
|
--
|
--
|
--
|
--
|
Inner city
|
23.02
(13.18, 32.85)
|
--
|
--
|
--
|
--
|
--
|
Urban
|
22.78
(13.97, 31.60)
|
-0.23
(-7.27, 6.80)
|
--
|
--
|
--
|
--
|
Aging suburb
|
23.75
(14.91, 32.59)
|
0.73
(-6.19, 7.66)
|
1.37
(-2.06, 4.79)
|
--
|
|
|
High-income suburb
|
24.93
(15.47, 34.39)
|
1.91
(-5.65, 9.48)
|
0.67
(-3.44, 4.78)
|
-0.70
(-4.18, 2.78)
|
--
|
|
Suburban edge
|
27.95
(18.61, 37.29)
|
4.94
(-2.41, 12.28)
|
3.99
(0.24, 7.74)
|
2.62
(-0.34, 5.58)
|
3.32
(0.03, 6.61)
|
--
|
Supermarket model: Estimated beta (95% confidence interval)
|
|
Urban core
|
Inner city
|
Urban
|
Aging suburb
|
High-income suburb
|
Suburban edge
|
Urban core
|
--
|
--
|
--
|
--
|
--
|
--
|
Inner city
|
1.87
(-2.50, 6.24)
|
--
|
--
|
--
|
--
|
--
|
Urban
|
1.41
(-2.51, 5.33)
|
-0.46
(-3.59, 2.67)
|
--
|
--
|
--
|
--
|
Aging suburb
|
0.18
(-3.75, 4.11)
|
-1.69
(-4.77, 1.39)
|
-1.23
(-3.12, 0.66)
|
--
|
--
|
--
|
High-income suburb
|
0.58
(-3.63, 4.78)
|
-1.30
(-4.66, 2.07)
|
-0.84
(-3.06, 1.38)
|
0.39
(-1.51, 2.30)
|
--
|
--
|
Suburban edge
|
1.68
(-2.46, 5.83)
|
-0.19
(-3.46, 3.08)
|
0.27
(-1.75, 2.29)
|
1.50
(-0.13, 3.13)
|
1.11
(-0.73, 2.94)
|
--
|
Bold indicates significant difference in percent of sit-down restaurants or percent of supermarkets across neighborhood type at the 0.05 significance level.
a Multivariable linear mixed effects regressions modeling the percent of sit-down restaurants relative to total sit-down restaurants and fast food restaurants and percent of supermarkets relative to total supermarkets, grocery stores and convenience stores as functions of neighborhood type in 1993, time elapsed since 1993, interaction between neighborhood type in 1993 and time elapsed, changes in residential population density, median household income, percent of white race and percent of single-family housing since 1993, total sit-down restaurants and fast food restaurants (sit-down restaurant model only), and total supermarkets, grocery stores and convenience stores (supermarket model only) and a random intercept for each neighborhood.
b Percent of sit-down restaurants relative to total sit-down restaurants and fast food restaurants in the neighborhood.
c Percent of supermarkets relative to total supermarkets, grocery stores and convenience stores in the neighborhood.
d Derived from cluster analysis of block-group level data from 1993: percent of population aged under 14, aged 15-29, 30-44, 45-64, and aged above 65, percent of education of college or above, percent of white race, percent of black race, median household income, residential population density, employment population density, mix of land use and percent of single-family housing.
Table 3 Contrastsa of percent of sit-down restaurantsb and percent of supermarketsc for each neighborhood typed pair in 2001
Sit-down restaurant model: Estimated beta (95% confidence interval)
|
|
Urban core
|
Inner city
|
Urban
|
Aging suburb
|
High-income suburb
|
Suburban edge
|
Urban core
|
--
|
--
|
--
|
--
|
--
|
--
|
Inner city
|
13.94
(6.04, 21.84)
|
--
|
--
|
--
|
--
|
--
|
Urban
|
18.14
(10.80, 25.48)
|
4.20
(-1.44, 9.85)
|
--
|
--
|
--
|
--
|
Aging suburb
|
19.51
(11.96, 27.06)
|
5.57
(-0.11, 11.26)
|
1.37
(-2.06, 4.79)
|
--
|
--
|
--
|
High-income suburb
|
18.81
(10.65, 26.96)
|
4.87
(-1.38, 11.13)
|
0.67
(-3.44, 4.78)
|
-0.70
(-4.18, 2.78)
|
--
|
--
|
Suburban edge
|
22.13
(14.04, 30.22)
|
8.19
(2.10, 14.28)
|
3.99
(0.24, 7.74)
|
2.62
(-0.34, 5.58)
|
3.32
(0.03, 6.61)
|
--
|
Supermarket model: Estimated beta (95% confidence interval)
|
|
Urban core
|
Inner city
|
Urban
|
Aging suburb
|
High-income suburb
|
Suburban edge
|
Urban core
|
--
|
--
|
--
|
--
|
--
|
--
|
Inner city
|
2.02
(-1.48, 5.51)
|
--
|
--
|
--
|
--
|
--
|
Urban
|
2.09
(-1.15, 5.34)
|
0.78
(-2.42, 2.58)
|
--
|
--
|
--
|
--
|
Aging suburb
|
0.32
(-3.03, 3.66)
|
-1.70
(-4.22, 0.82)
|
-1.78
(-3.29, -0.26)
|
--
|
--
|
--
|
High-income suburb
|
1.91
(-1.70, 5.52)
|
-0.11
(-2.88, 2.66)
|
-0.19
(-2.00, 1.63)
|
-0.25
(-1.91, 1.40)
|
--
|
--
|
Suburban edge
|
1.84
(-1.74, 5.41)
|
-0.18
(-2.88, 2.52)
|
-0.19
(-2.00, 1.63)
|
1.59
(0.05, 3.13)
|
1.52
(0.21, 2.84)
|
--
|
Refer to the legends in Table 2.
Table 4 Contrastsa of percent of sit-down restaurantsb and percent of supermarketsc for each neighborhood type d pair in 2011
Sit-down restaurant model: Estimated beta (95% confidence interval)
|
|
Urban core
|
Inner city
|
Urban
|
Aging suburb
|
High-income suburb
|
Suburban edge
|
Urban core
|
--
|
--
|
--
|
--
|
--
|
--
|
Inner city
|
2.59
(-7.69, 12.86)
|
--
|
--
|
--
|
--
|
--
|
Urban
|
12.33
(2.93, 21.74)
|
9.75
(2.49, 17.01)
|
--
|
--
|
--
|
--
|
Aging suburb
|
14.21
(4.67, 23.74)
|
11.62
(4.46, 18.79)
|
1.87
(-2.56, 6.30)
|
--
|
--
|
--
|
High-income suburb
|
11.15
(1.09, 21.21)
|
8.57
(0.87, 16.26)
|
-1.18
(-6.47, 4.11)
|
-3.05
(-7.65, 1.54)
|
--
|
--
|
Suburban edge
|
14.85
(5.04, 24.67)
|
12.27
(4.95, 19.57)
|
2.52
(-2.14, 7.17)
|
0.65
(-3.18, 4.47)
|
3.70
(-0.60, 8.00)
|
--
|
Supermarket model: Estimated beta (95% confidence interval)
|
|
Urban core
|
Inner city
|
Urban
|
Aging suburb
|
High-income suburb
|
Suburban edge
|
Urban core
|
--
|
--
|
--
|
--
|
--
|
--
|
Inner city
|
2.19
(-2.37, 6.76)
|
--
|
--
|
--
|
--
|
--
|
Urban
|
2.95
(-1.24, 7.13)
|
0.75
(-2.48, 3.98)
|
--
|
--
|
--
|
--
|
Aging suburb
|
0.48
(-3.76, 4.72)
|
-1.71
(-4.90, 1.47)
|
-2.46
(-4.43, -0.50)
|
--
|
--
|
--
|
High-income suburb
|
3.57
(-0.89, 8.04)
|
1.38
(-2.04, 4.80)
|
0.63
(-1.72, 2.98)
|
3.09
(1.05, 5.14)
|
-
|
--
|
Suburban edge
|
2.03
(-2.32, 6.39)
|
-0.16
(-3.42, 3.10)
|
-0.91
(-2.98, 1.16)
|
1.55
(-0.15, 3.26)
|
-1.54
(-3.46, 0.38)
|
--
|
Refer to the legends in Table 2.
Figure 2 Estimated mean a of percent of sit-down restaurants by six types of neighborhoods b
a Multivariable mixed effects regression modeling percent of sit-down restaurants relative to total sit-down restaurants and fast food restaurants in each neighborhood as a function of neighborhood type in 1993, time elapsed since 1993, interaction between neighborhood type in 1993 and time elapsed, changes in residential population density, median household income, percent of white and percent of single-family housing since 1993, total sit-down restaurants and fast food restaurants and a random intercept for each neighborhood.
b Derived from cluster analysis of block-group level data in 1993: percent of age under 14, age aged 15-29, 30-44, 45-64, and aged above 65, percent of education of college and above, percent of white, percent of black, median household income, residential population density, employment population density, mix of land use and percent of single-family housing.
Figure 3 Estimated mean a of percent of supermarkets relative to total supermarkets, grocery stores and convenience stores by six types of neighborhoods b: Twin Cities Region, 1993-2011.
a Multivariable mixed effects regression modeling percent of supermarkets relative to total supermarkets, grocery stores and convenience stores in each neighborhood as a function of neighborhood type in 1993, time elapsed since 1993, interaction between neighborhood type in 1993 and time elapsed, changes in residential population density, median household income, percent of white and percent of single-family housing since 1993, total supermarkets, grocery stores and convenience stores and a random intercept for each neighborhood.
b Derived from cluster analysis of block-group level data in 1993: percent of age under 14, aged 15-29, 30-44, 45-64, and aged above 65, percent of education of college or above, percent of white, percent of black, median household income, residential population density, employment population density, mix of land use and percent of single-family housing.
3.4 Sensitivity Testing
Tables S7-S8 in the Additional File 1 contain regression results using the census tract and place to measure food availability. Tract models generated similar results to the major results based on Block Group. But Place models showed inconsistencies, particularly for the sit-down restaurant model. Urban and aging suburb neighborhoods experienced lower increases in the percent of sit-down restaurants in the Block Group model, whereas we failed to observe such a difference in the Place model. Similarly, urban core had higher percent of sit-down restaurants than inner city in 1993 in the Block Group and Tract models, but the Place models did not show such a difference. Interestingly, high-income suburb and suburban edge had lower percent of sit-down restaurants than inner city in 1993 in the Place model but no difference in the Block Group or Tract model. The differences between the Block Group, Tract and Place models suggested that the measure of relative availability was sensitive to spatial unit. Because the size of a census block group was not always small (varying from 0.04 in the urban core to 154.19 km2 in the suburban edge with median and interquartile range values of 0.88 and 1.63) and actually increased with the distance to urban core increases, we argued that our measure of relative food availability was a reasonable small-area measure.