Estimating climate change and mental health impacts in Canada: A cross-sectional survey protocol

Nunavut, since each household address in the Canada Post database does not include a corre- sponding telephone number. Instead, we will send one additional reminder letter to random households that have not completed the survey. Initially, the sampling frame for telephone recruitment will include 60 telephone numbers per province/territory, assuming a 10% response rate. Similar to our approach to our mail- based sampling frame, we will monitor the response rates for each province and territory, as well as for urban and rural areas, and we will make adjustments to the sample parameters and the number of randomly selected telephone numbers accordingly. Within the first 10 days of each month, we will dial each of the selected phone numbers at least once. Phone numbers will be dialled on the following days and times in the time zone of the area being surveyed: Monday to Friday between 13:00 to 20:00; Saturday between 10:00 to 18:00; and Sunday between 12:00 to 20:00. No initial calls will be made on statutory holidays. Each call attempt will allow for a minimum of five rings or until it is sent to voicemail. Initially, we will attempt each telephone number up to five times over the course of one month. Each call will be made on different days of the week and during different times of the day. A voicemail message will be left on the sec- ond unsuccessful attempt and every unsuccessful attempt thereafter. Eligible participants can complete the questionnaire over the telephone with a trained interviewer; schedule an appoint-

ment to be called back later; or receive a link by email or text message to complete the ques- tionnaire online. All callback appointments will be scheduled for the day and time requested by the participant. Regardless of the recruitment method (mail or telephone), participants will complete the same questionnaire either through Voxco via Computer Assisted Telephone Interviewing (CATI) or using an online survey instrument. However, the introductory recruitment script will differ depending on the mode of recruitment: the telephone version requires more intro- ductory information as well as the questions that are necessary to randomly select the member of the household to interview (i.e. the person 13 years or older in the household who has the next birthday). For both modes of recruitment, the day that the questionnaire is completed will be logged as the reference day (i.e. the day when the last question was completed and the survey was submitted). To achieve a representative sample, we will aim for the number of completed questionnaires to be proportional to the distribution of age groups (13–34 years; 35–54 years; 55–74 years; 75 + years) in each province/territory based on Statistics Canada data (2021) [48] (Table 3). Addi- tionally, within each province/territory, we will aim for the number of questionnaires com- pleted to be proportional to that province/territory’s urban/rural population sizes based on the first two digits of respondents’ postal codes based on Statistics Canada data (2016) [49].

We will use the Market Research and Intelligence Agency (MRIA) empirical method to cal- culate the questionnaire response rate [50]. Thus, we will divide the number of responding units by the sum of in-scope and unresolved units. The MRIA empirical method is widely used in Canada, including in previous surveys conducted by the Government of Canada [47, 50]. Options to overcome recruitment challenges If any age or geographical targets are not met during the course of the survey administration, we will employ several methods to increase recruitment success. For example, the proportion of cellular phone, landline telephone, and mailout recruitments in the sample can be adjusted. For those participants contacted by cellular telephone, if there is no response after the third attempt, one text message may be sent to the cellular phone containing a link to the online ver- sion of the questionnaire. Small incentives (i.e. gift cards) may be used to increase enrolment for subpopulations that might have lower response rates (e.g. those living in the Territories PLOS ONE Climate change and mental health survey protocol PLOS ONE | https://doi.org/10.1371/journal.pone.0291303 October 11, 2023 7 / 12