The data

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The prevalence of sexual intercourse and condom use and the number of sexual partners among 15- to 19-year-olds were estimated from the 1996/1997 National Population Health Survey (NPHS) cross-sectional file and from cycles 2.1 and 3.1 of the Canadian Community Health Survey (CCHS).

Respondents to the cross-sectional component of the NPHS were surveyed from June 1996 to August 1997, with an overall response rate of 83%. The sample aged 15 to 19 for the questions about sexual intercourse and age at first intercourse numbered 4,449; the analyses of multiple partners were based on a sample of 3,600. Both samples were weighted to represent a population of approximately 2.1 million. Virtually all the NPHS data (98.5%) were collected in telephone interviews. Details of the design and sampling techniques have been described elsewhere.12

Cycle 2.1 of the CCHS was conducted from January to December in 2003; cycle 3.1, from January to June in 2005. The response rates for cycle 2.1 and cycle 3.1 were 81% and 79%, respectively. The samples used for this article numbered 11,022 and 9,874 respondents aged 15 to 19, weighted to represent populations of approximately 2 million and 2.1 million, respectively. Telephone interviews accounted for 70% of all cycle 2.1 interviews, and 61% of cycle 3.1 interviews. Details of the CCHS design and sampling techniques have been published elsewhere.13 

All differences were tested to ensure statistical significance, which was established at the 0.05 level. To account for survey design effects, standard errors and coefficients of variation were estimated using the bootstrap technique.14, 15

The percentage of 15- to 19-year-olds who had had sexual intercourse at least once was based on yes/no responses to the following question in the NPHS and in cycles 2.1 and 3.1 of the CCHS: "Have you ever had sexual intercourse?" Respondents were asked how old they were "the first time," which was used to calculate the proportions who became sexually active at age 14 or younger, and at ages 15 and 16. The percentage who had had sexual intercourse with more than one person in the past year was based on the number of partners respondents said they had had in the previous 12 months. Estimates of condom use among sexually active young people who had been with multiple partners in the past year and/or who were not married or not in a common-law relationship were based on responses to: "Did you use a condom the last time you had intercourse?" Because condom use referred to the most recent sexual encounter, it does not necessarily reflect typical behaviour. It is also possible that the question itself generated confusion among respondents, as it does not specifically ask about male versus female condom use.

The mode of data collection can influence response rates, data quality and non-sampling errors, notably bias.16, 17 "Social desirability," which has been identified as a source of bias in the CCHS,13 may be especially relevant to this analysis. Social desirability refers to a tendency for respondents to modify their answers in an effort to construct a more favourable image of themselves. Some studies have found that face-to-face interviews are more susceptible to this type of bias than are telephone interviews, while others have found that respondents interviewed by telephone are less likely to share or accurately report potentially embarrassing attitudes or behaviours.18, 19 Thus, social desirability has the potential to result in both over- and underestimation of a particular behaviour, since the perception of what is "desirable" may differ depending on a respondent's age, gender and socio-economic status. Some teens may give socially acceptable answers (under-report) to questions about sexual behaviour; others may exaggerate their sexual experience.

Differing proportions of the interviews in three surveys used for this analysis were conducted by telephone. To ascertain the impact of these differences, analyses were rerun for the subset of respondents from each survey who were interviewed by telephone. The results were generally the same as those for the full sample (data not shown), thereby lending support to the stability and credibility of the original estimates.

The term "sexual intercourse" was not defined in the NPHS or CCHS questions, so it is possible that some teens may have misinterpreted the question. As well, recall errors are possible.