The Economic Burden of Asthma in Canada

Phase A: This phase will be a prospective cohort study of randomly chosen residents of two census subdivisions of BC with a self-reported physician diagnosis of asthma. The study participants will be recruited via landline and cell phone-based random digit dialing across two regions in British Columbia: the Census Sub-Division of the city of Vancouver and the Census Division of Central Okanagan. The target area was chosen based on Census Sub-Division as it provides a clear definition of the target population for whom demographic and socioeconomic data is available through national and provincial census and surveys. The Okanagan Census Sub-Division was particularly chosen as it provides data on both urban and rural populations (18.6% rural population in 2006). According to 2006 census profile, the population of these two Census Sub-Divisions is, respectively, 578,041 and 162,276. Accordingly, about 78% of participants would be recruited from Vancouver, and 22% from Okanagan. The study will randomly sample participants from all the area codes in these regions. The target population will include household residents residing in telephone exchanges within the target Census Sub-Divisions. Random digit dialing will be geographically stratified with an associated sample weight that will approximate a probability sample of the individuals in the population. Previous studies have shown that acceptably high response rates can be achieved among Canadian participants with asthma.

Random digit dialing samples have been shown to be more representative than population samples drawn using alternative methods such as telephone directories or electronic white pages. However, random digit dialing is not free of bias. Non-random non-response is an issue, and it has been shown that reducing the non-response rate may directly reduce non-response bias. Non-responders in random digit dialing may not have been contacted because the non-responders work multiple minimum-wage jobs. The investigators will place multiple calls and leave messages on answering machines. Random digit dialing is also subject to no-telephone bias - households without telephones are out of reach by random digit dialing, and the households without telephones probably differ in socioeconomic status than the overall population. However, for the urban, suburban, and semi-rural sub-populations within British Columbia, telephone coverage is almost universal.

Inclusion of cell phones in random digit dialing is an attempt to mitigate such biases and increase the fraction of population covered by sampling. More importantly, the inclusion of cell phones is likely to result in inclusion of participants that might be absent from landline random digit dialing (younger and more educated groups) thus mitigating the sampling bias of random digit dialing. Cell phone random digit dialing is an active area of research and it is too soon to know with confidence what should and should not be regarded as a "Best Practice". For inclusion of cell phones in random digit dialing, the investigators will use the "Screening Approach". In this approach the interview is only conducted with participants sampled via the cell phone frame who do not have a landline, thus excluding numbers from the cell phone sample in the overlap (i.e., screening out the participants with both a cell phone and a landline). In this alternative sampling design, participants who have at least one household landline telephone and use at least one cell phone would be eligible for inclusion only from the landline frame. This decreases the bias due to higher probability of sampling from households who have both a landline and cell phone.

The study will employ the Waksberg method of random digit sampling. In this method, all telephone numbers are initially split into blocks of equal size, called "primary sampling units". The primary sampling unit will be a mixture of landlines and cell phones. One randomly selected telephone number is called from each selected primary sampling unit. When a residential connection is reached in the first stage, the primary sampling unit qualifies for inclusion in the second stage.

Respondents will be asked the following question: "Is there a member of the household between ages of 1 to 85 who has had asthma ever diagnosed by a physician". If the response is affirmative the study assistant will ask if the participant in question can continue the conversation directly (parents in case of children). Eligible participants (or in the case of children their parents or guardians) will be informed of the objectives of the study, verbally consented, and then screened for eligibility based on age and history of asthma. Participants who meet eligibility criteria and verbally consent over the phone will be asked to come to the study laboratory (one at Vancouver and one at Kelowna, corresponding to the two Census Sub-Divisions) and the participant will receive a detailed explanation of the study and the study consent form at that time.