Table 5

Aggregation errors in measures of the accessibility of hospitals at the census tract level

Absolute difference between accessibility measure obtained from CTCa and WBlc aggregation methods

Percentiles

Mean

5%

10%

25%

50%

75%

90%

95%


Shortest network distance

Minimum network distance

365.35

11.19

22.54

57.40

147.23

365.04

947.76

1,595.45

Average distance to 3 closest services

284.19

5.88

12.07

32.04

102.51

276.01

719.27

1,250.98

Average distance to 5 closest services

290.26

5.10

11.81

34.16

94.54

275.32

800.69

1,300.49

Average distance to all services

309.02

3.44

7.42

26.94

89.12

275.50

869.61

1,470.33

Number of services within 500 metres

0.04

0.00

0.00

0.00

0.00

0.00

0.06

0.34

Number of services within 1000 metres

0.07

0.00

0.00

0.00

0.00

0.00

0.26

0.50

Number of services within 2000 metres

0.14

0.00

0.00

0.00

0.00

0.15

0.47

0.77

Shortest time distance

Minimum time distance

0.37

0.02

0.03

0.09

0.20

0.41

0.84

1.34

Average distance to 3 closest services

0.31

0.01

0.02

0.06

0.15

0.34

0.73

1.11

Average distance to 5 closest services

0.31

0.01

0.02

0.06

0.15

0.35

0.75

1.16

Average distance to all services

0.32

0.02

0.03

0.06

0.15

0.37

0.73

1.22

Number of services within 10 minutes

0.69

0.00

0.00

0.07

0.39

0.93

1.72

2.60

Number of services within 20 minutes

0.76

0.00

0.00

0.00

0.25

0.71

1.98

3.20

Number of services within 30 minutes

0.52

0.00

0.00

0.00

0.04

0.41

0.98

2.15


Absolute difference between accessibility measure obtained from WDAb and WBlc aggregation methods

Mean

5%

10%

25%

50%

75%

90%

95%


Shortest network distance

Minimum network distance

134.15

3.51

6.89

20.20

51.74

136.36

311.82

501.59

Average distance to 3 closest services

118.06

2.14

4.50

16.10

46.43

126.23

275.19

413.62

Average distance to 5 closest services

123.87

2.15

5.01

14.99

48.60

140.36

300.31

421.81

Average distance to all services

157.70

1.95

4.05

17.45

66.96

240.45

360.42

473.07

Number of services within 500 metres

0.02

0.00

0.00

0.00

0.00

0.00

0.03

0.12

Number of services within 1000 metres

0.04

0.00

0.00

0.00

0.00

0.00

0.08

0.22

Number of services within 2000 metres

0.08

0.00

0.00

0.00

0.00

0.05

0.22

0.40

Shortest time distance

Minimum time distance

0.12

0.00

0.01

0.02

0.06

0.11

0.27

0.43

Average distance to 3 closest services

0.10

0.00

0.01

0.02

0.05

0.10

0.23

0.39

Average distance to 5 closest services

0.10

0.00

0.01

0.02

0.04

0.10

0.22

0.39

Average distance to all services

0.10

0.00

0.01

0.02

0.04

0.10

0.22

0.39

Number of services within 10 minutes

0.23

0.00

0.00

0.02

0.12

0.29

0.57

0.84

Number of services within 20 minutes

0.22

0.00

0.00

0.00

0.06

0.20

0.44

0.85

Number of services within 30 minutes

0.14

0.00

0.00

0.00

0.01

0.11

0.31

0.68


a Aggregation method based on census tract centroid (the least accurate method).

b Aggregation method based on the population-weighted mean of the accessibility measure for dissemination areas within census tracts.

c Aggregation method based on the population-weighted mean of the accessibility measure for blocks within census tracts (the most accurate method).

Apparicio et al. International Journal of Health Geographics 2008 7:7   doi:10.1186/1476-072X-7-7

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