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The data are from the 2007 Canadian Community Health Survey (CCHS), which covers the household population aged 12 years or older. Residents of Indian reserves, institutions and some remote areas; full-time members of the Canadian Forces; and all residents (military and civilian) of Canadian Forces bases were excluded. Interviews were conducted from January through December, 2007. The overall response rate was 78%, yielding a sample of 65,946 respondents. More information about the CCHS is available in a published report5 and on Statistics Canada's Web site.

This study was based on the population aged 20 years or older and represents 57,367 respondents who answered the question on television viewing, and 57,617 respondents who answered the question on leisure-time computer use.

All estimates were weighted to be representative of the household population aged 20 years or older in 2007. Cross-tabulations and logistic regression analysis were used to study associations between socio-demographic characteristics and self-reported screen time. To account for the survey design effect, standard errors, coefficients of variation and 95% confidence intervals were estimated using the bootstrap technique.6,7 Differences between estimates were tested for statistical significance, which was established at p < 0.05.

Screen time was assessed by asking CCHS respondents the number of hours in a typical week over the past three months they spent watching television (including videos) and using a computer (including playing computer games and using the Internet). Respondents were instructed to report leisure-time hours only and to exclude time spent on these activities at work or school. For each behaviour, respondents reported their weekly hours in one of eight categories: none, less than 1 hour, 1 to 2 hours, 3 to 5 hours, 6 to 10 hours, 11 to 14 hours, 15 to 20 hours, or more than 20 hours. No guidelines have been proposed for adults, but the Canadian Paediatric Society recommends a maximum of two hours of television per day for children.8 Among adults, a variety of cut-points have been used in the literature to define frequent viewing. For this analysis, those who reported 15 or more hours per week were defined as frequent television viewers, and those who reported 11 or more hours of leisure-time computer use were defined as frequent computer users. To calculate the proportion of total screen time devoted to computers, continuous measures were derived for television viewing and computer use by assigning the midpoint of each response category (0, 0.5, 1.5, 4, 8, 12.5, 17.5, or 25 hours for the highest category).

Based on their highest level of education, respondents aged 25 years or older were grouped into four categories: postsecondary graduation, some postsecondary, secondary graduation, and less than secondary graduation. The same categories were used for those aged 20 to 24 years, but for these respondents, education was based on the highest level in the household.

Household income groups were derived by calculating the ratio between the total household income from all sources in the previous 12 months and Statistics Canada's low-income cutoff (LICO) specific to the number of people in the household, the size of the community, and the survey year. These adjusted income ratios were grouped into quintiles (five groups, each containing one-fifth of Canadians).

Trends in television viewing are from the General Social Survey (GSS) (1986 and 2005), which used a one-day time use diary to collect information on time spent on a wide variety of activities.3

CCHS estimates of screen time are based on self-reported data, which are subject to social desirability and recall biases. Single-item measures for the assessment of sedentary behaviours lack content validity and likely yield only crude estimates.9 Comparisons with GSS data suggest that television viewing time is underestimated in the CCHS ; according to 2005 data from the GSS, the prevalence of frequent television viewing (15 or more hours per week) was 39%, substantially above the estimate of 29% from the 2007 CCHS.