Thanks James and Miles, I appreciate your summary at the start of this post. Good to read this and important work to attempt to validate the core aims of the conference. Couple of ideas for survey tweaks:

1. Push to get higher response rate for control group, too. This should give a more representative sample of non-attendees. A 5% response rate is probably the self-selected “keen beans” who may be more likely to stay the course in their views and actions (rather than decay over 6 months), and who may just be too busy for that particular year’s EAGx.
2. Add (or adjust) a survey step much later than 6 months. In my view, 6 months is too short to lose social connections for the control group. Such connections are lost over longer periods (1.5+ years), and especially less likely to be lost by the “keen beans” who respond to the survey. These may be more likely to be conscientious or proactive respondents who stay in touch with connections more often.