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On 15 Jul 2017, at 16:25, Leslie <geekgirl@gmail.com> wrote:

You will also note that I proposed actively surveying former
participants specifically to counter the risk of survivorship bias.

As I mentioned above, that is very difficult. How would you identify and reach out to the people who have left and don't feel comfortable joining?

I don't see any particular additional difficulty in contacting those that recently participated but no longer do so, using the contact details they provided at registration, and ask them why (and whether cessation was due to a negative experience). I'm not talking about reaching back 20 years.