To the Editors;
In the November 2007 issue of Paediatrics & Child Health, Bruce et al address an important issue in their article titled “The relationship between childhood behaviour disorders and unintentional injury events”. They test their hypothesis that children with behaviour disorders have an increased risk of unintentional injuries. They do this through analysis of linked administrative health and community services databases for hospital admissions, physician visits and medication prescriptions. The authors note a limitation in one of the databases used, with which I agree. Children and families represented in the Community Services Family Benefits and Pharmacare database either receive financial assistance or social work support and tend to be single-parent families with lower levels of income and education than the general population. The authors compare them with children from administrative databases that include virtually the entire population of Nova Scotia. The possible bias is that such children are at higher risk for injury for reasons having nothing to do with the presence or absence of behaviour disorders. While noting this important limitation, the authors did not adjust for it other than controlling for age and sex, nuisance variables that need to be controlled but that have no relevance to the potential biases that could account for the relationships found. This could be addressed in two ways by further analysis. The first would be to choose a control group as comparable as possible to their community services research participants. This control group could simply be children from the same Community Services database who require a prescription for another reason, for example, asthma or infection, and who do not meet their criteria for a behaviour disorder. A second, less desirable, option would be to use the province-wide health databases but match the children with behaviour disorders to controls by age, sex and socioeconomic proxies, such as postal code, a marker that is correlated with income. Odds ratios using these proposed groups would be more convincing than the reported odds ratios because the latter rely on groups being comparable for characteristics other than the presence or absence of behaviour disorders.