The methods of sample selection and examining procedures have been described in detail . In brief, a sample of 18,000 randomly selected telephone numbers for the City of Winnipeg was obtained. From March 1993 to May 1994, interviewers systematically called these numbers, and following an appropriate introduction, they determined whether the number was residential and whether the household contained eligible individuals — those aged 20 to 44 years. If nobody answered, calls were repeated up to seven times, at varying times on varying days. If more than one household member was eligible, the interviewer used a predetermined sampling scheme to select one subject. The subject’s name and postal address were recorded. This procedure identified 3402 age-eligible individuals.
The selected individuals were sent a questionnaire with a cover letter explaining the purpose of the study as a survey of general respiratory health and not specifically of asthma. After three weeks, nonresponders were sent a second questionnaire; they were phoned to encourage them to complete it, either by mail or over the telephone. Responses were obtained from 2981 of the selected individuals (84.8%).
Questions were based on the European Community Respiratory Health Survey questionnaire . The present comparison focused on responses to two questions: “Have you had an attack of asthma in the last 12 months?” and “Are you currently taking any medication (including inhalers, aerosols or tablets) for asthma?”. Answers to questions regarding asthma symptoms in the last 12 months were also analyzed: wheezing or whistling in the chest, being wakened by a feeling of tightness in chest, attacks of shortness of breath or coughing. Finally, respondents were questioned about a diagnosis of asthma by a physician at any time in their lives.