Intervention
The mailout for the intervention groups included an email approach for the second reminder. Both the intervention and control group were approached four times: the initial invitation plus three reminders. In the control group all four approaches used a mailed paper letter sent to each participant’s work address. All survey materials are available at The intervention group only differed at the second reminder, where they were approached by email and could only complete online, receiving no paper letter or paper copy of the survey. The comparison between the intervention and control group therefore includes a different method of approach and a different method of completion: email approach plus online completion versus mailed approach plus a choice of online or paper completion. The email included the same text as the paper letter plus a link and instructions for online completion. Emails were sent to email addresses from the AMPCo database or from email addresses provided by participants in earlier waves of the survey.