Abstract
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Background
Postal codes are often the only geographic identifier available for assigning contextual or environmental information to a study population. This analysis assesses the influence of three factors—delivery mode type (mode of postal delivery), representative point type (source of latitude-longitude coordinates), and community size—on the accuracy of postal code spatial assignment.
Data and methods
PCCF+ (Postal Code Conversion File Plus) was used to assign delivery mode type, representative point type and community size to each individual in the 2011 Census of Canada. A sample (n = 1,004) was randomly selected with a minimum of 90 observations for each category of those three factors. Based on the address information of individuals in the sample, measures of positional accuracy for geocoding from residential postal codes (PCCF+) versus reference locations as determined by full street addresses (Google Maps) were calculated using a geographic information system. Accuracy was measured as the distance that the geocoded position differed from the full street address.
Results
Positional accuracy was related primarily to mode of postal delivery. Rural and mixed (partly urban, partly rural) modes had much higher geocoding error than did urban modes. Rural and small-town Canada and latitude and longitude based on dissemination area centroids had low accuracy, largely because of their close relationship to rural and mixed modes of delivery.
Interpretation
The accuracy of geocoding from postal codes can vary. Geocoding imprecision may result in misclassification, depending on the spatial resolution of the environmental or contextual measures. The spatial resolution required for a study helps to identify subpopulations that should be excluded because of inadequate positional accuracy.
Keywords
Community size, delivery mode type, misclassification, representative point type, street address
Findings
Geocoding based on a full street address is a highly accurate way of assigning geographic coordinates to an individual’s residential location. However, address information is usually not available. As well, for many types of population health research, such precision is not necessary. [Full Text]
Authors
Saeeda Khan (Saeeda.khan@canada.ca), Lauren Pinault and Michael Tjepkema are with the Health Analysis Division at Statistics Canada, Ottawa, Ontario. Russell Wilkins is with the Department of Epidemiology and Community Medicine at the University of Ottawa.
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What is already known on this subject?
- In Canada, postal codes are often the only geographic identifier available for assigning contextual or environmental information to a study population.
- The Postal Code Conversion File (PCCF) and Postal Code Conversion File Plus (PCCF+) provide the means to geocode datasets using only postal codes.
- The literature on geocoding based on postal codes shows much less error in urban areas than in rural areas.
What does this study add?
- Using self-reported address data from the 2011 Census of Population, this study compares the positional accuracy of geocoding based on postal codes versus full street addresses.
- All postal codes in the census were processed through PCCF+ Version 6C; positional accuracy was measured by the distance by which the geocoded position differed from the street address.
- Positional accuracy based on postal codes is primarily dependent on delivery mode type.
- The best accuracy was for large urban apartment buildings, followed by ordinary urban households.
- Accuracy generally improved with increasing community size.
- Rural and small-town postal codes, along with post office boxes, had the highest levels of error.
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