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Table 5 Aggregation errors in measures of the accessibility of hospitals at the census tract level

From: Comparing alternative approaches to measuring the geographical accessibility of urban health services: Distance types and aggregation-error issues

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 WDA b and WBl c 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
  1. a Aggregation method based on census tract centroid (the least accurate method).
  2. b Aggregation method based on the population-weighted mean of the accessibility measure for dissemination areas within census tracts.
  3. c Aggregation method based on the population-weighted mean of the accessibility measure for blocks within census tracts (the most accurate method).