Introduction : Individuals with intellectual disabilities have a higher prevalence of health problems, including psychiatric and behavioural conditions, than the general population. However, there is little population-based information in Canada about individuals with a dual diagnosis of psychiatric disorder and intellectual impairment. The aim of this study was to determine whether the 2005 Canadian Community Health Survey ( CCHS ) and the 2006 Participation and Activity Limitation Survey ( PALS ) could be used to estimate the prevalence of dual diagnosis in Canada.

Methods : We undertook a secondary analysis of two population-based surveys to determine if these could be used to estimate the prevalence of psychiatric or behavioural conditions among adults with intellectual disabilities in Canada.

Results : The surveys reflect prevalence estimates of intellectual disabilities ( CCHS : 0.2% and PALS : 0.5%) that are considerably lower than those published in the literature. While it was possible to calculate the proportion of individuals with a dual diagnosis ( CCHS : 30.6% and PALS : 44.3%), the surveys were of limited use for detailed analyses. The estimates of prevalence derived from the surveys, especially from the CCHS , were of unacceptable quality due to high sampling variability and selection bias.

Conclusion : The estimates should be interpreted with caution due to concerns regarding the representativeness of the sample with intellectual disabilities in the national surveys.