I've a:

• "Retrospective panel survey": In each year all units are asked "who (X) first told you about us (in the year you first learned about us)?"
• There is lots of attrition from the panel, which may vary by X, as well as new people entering in each year

My question of interest: How is X, the 'how did you first learn about us' thing changing across time? I.e., is the 'point of first contact' (referrer) changing from year to year?

Possible approaches

A. Single-retrospect:

If I use only the most recent (2020) retrospective data this may lead to a bias from differential attrition related to X (as well as issues of imperfect recall).

If people who 'heard about us through Spaghetti Monster' have dropped out at twice the average rate, and Spaghetti Monster was the referrer for 1/2 of those who learned about us in 2015. ...we will falsely report that "only 1/4 of people who heard about us in 2015 heard about us through SM".

B. Recent-retrospect for each year

I could look instead at the 2016 recall data only for 2015, 2017 data for 2016 etc., as there will be less attrition between the shorter time intervals.

But this has its own problem: the share who respond to the survey from coming from each referrer fluctuates from year to year. Suppose in 2020 there is a particularly low SM response rate vs 2019 ... we would falsely claim that SM-referrals fell dramatically in 2020 relative to 2019. This should not be a problem for the single retrospect

Vaguely remember that I've seen papers dealing with similar issues but I can't recall. Before I try to reinvent the wheel, any suggestions? Thanks!