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

To develop a sampling frame for mail recruitment, we will draw from a national list of addresses maintained by Canada Post. Canada Post will provide a new sampling frame every two months by randomly selecting addresses to meet our sample parameters. From this sam- pling frame, we will randomly select addresses and we will mail survey invitations as addressed letters to those addresses. Addressed letters will be sent monthly and will contain instructions for completing the questionnaire. To develop a sampling frame for telephone recruitment, we will obtain a national list of landline and cellular telephone numbers from ASDE Inc., a survey sample company that pur- chases this information from phone companies. A new sampling frame will be developed each month. Recruitment We will use two different modes of recruitment: an addressed letter sent by postal mail and a telephone call (landlines and cellular). Ideally, participants will only be contacted by one mode; however, some overlap between mail and telephone sampling is theoretically possible as these samples will be drawn from separate but not mutually exclusive databases, although any overlap is statistically improbable. The modes of recruitment will be initially allocated as fol- lows: 75% for mail recruitment and 25% for telephone recruitment. Participants recruited via addressed letter will be directed to a website containing the online questionnaire, or they can phone (toll-free) to complete the questionnaire orally. Initially, we will mail 180 addressed letters in each province and territory, assuming an estimated 10%

response rate. We will monitor the response rates for each province and territory, as well as for urban and rural areas using the first two digits of respondents’ postal codes, and we will adjust the sample parameters and the number of randomly selected addresses accordingly. Each selected household will only receive one addressed letter during the study period, with the exception of households in Nunavut. In all other provinces and territories, our address-based sample will include a household address and corresponding telephone number; thus, we will be able to follow up on mailed survey invitations with a reminder telephone call, which will be made two weeks after the addressed letters are mailed, provided that selected participants have not completed the survey. Reminder telephone calls will not be possible for households in Fig 1. An overview of the multi-stage random sampling method. https://doi.org/10.1371/journal.pone.0291303.g001 PLOS ONE Climate change and mental health survey protocol PLOS ONE | https://doi.org/10.1371/journal.pone.0291303 October 11, 2023 6 / 12