Analyzing the effects of place on injury: Does the choice of geographic scale and zone matter?

BACKGROUND

Recent studies have shown that the morbidity and mortality associated with injury of pedestrians are inversely related to socio-economic status (SES). However, in drawing inferences from this association, investigators have paid little attention to the modifiable artifacts related to scale and how the data are partitioned. The purpose of this population-based study was to identify the relation between SES and incidence patterns of pedestrian injury at 4 different geographic scales.

METHODS

We used a Poisson generalized linear model, stratified by age and sex, to analyze the relation between each of 4 area measures of SES and incidence patterns of pedestrian injuries occurring in metropolitan Vancouver between 1 January 2001 and 31 March 2006. The 4 area measures of SES were based on boundaries of dissemination areas, census tracts, custom-defined census tracts (generated by reassignment of dissemination area boundaries by means of a geographic information system) and census subdivisions of the Canadian census. We measured the SES of the location where the injury occurred with the Vancouver Area Neighbourhood Deprivation Index.

RESULTS

A total of 262 injuries in adults (18 years of age or older) were analyzed. Among adult men, the odds ratio (OR) for injury of pedestrians at the scale of dissemination area was 4.93 (95% confidence interval [CI] 2.89-8.42) for areas having the lowest SES relative to those with the highest SES. For the same population, the OR for injury was lower with increasing aggregation of data: 2.33 (95% CI 1.45-3.74) when census tracts were used, 3.26 (95% CI 2.06-5.16) when modified census tracts were used and 1.27 (95% CI 0.47-3.45) when census subdivisions were used. Among adult women, the OR for pedestrian injury by SES was highest at the scale of census subdivision within medium-low SES areas (4.33, 95% CI 1.23-15.22). At the census subdivision scale, the relation between SES and incidence pattern of injury was not consistent with findings at smaller geographic scales, and the OR for injury decreased with each increase in SES.

INTERPRETATION

In this analysis, there was significant variability when different administrative boundaries were applied as proxy measures of the effects of place on incidence patterns of injury. The hypothesized influence of SES on prevalence of pedestrian injury followed a statistically significant socio-economic gradient when analyzed using small-area boundaries of the census. However, researchers should be aware of the inherent variability that remains even among the more homogenous population units.