I’m not sure your interpretation of the 80% number is correct. As you originally presented it, 80% of people becoming vegetarian stop within a few years.
However:
Sampling the population at a given time (e.g. the time the facebook group has been running) will be more likely to catch long-term vegetarians, because they make up a much greater proportion of vegetarian-years.
Facebook hasn’t been going that long and I don’t think has reached equilibrium. You may have collected a reasonable number of people in the couple of years after they joined the group, who would eventually be recidivists but aren’t yet (this could actually be a pretty good group to target).
Possible selection effects will mean even at a given time long-term vegetarians are more likely to join a facebook group, if it’s more of a core part of their identity (this could also go the other way).
I’m not sure your interpretation of the 80% number is correct. As you originally presented it, 80% of people becoming vegetarian stop within a few years.
However:
Sampling the population at a given time (e.g. the time the facebook group has been running) will be more likely to catch long-term vegetarians, because they make up a much greater proportion of vegetarian-years.
Facebook hasn’t been going that long and I don’t think has reached equilibrium. You may have collected a reasonable number of people in the couple of years after they joined the group, who would eventually be recidivists but aren’t yet (this could actually be a pretty good group to target).
Possible selection effects will mean even at a given time long-term vegetarians are more likely to join a facebook group, if it’s more of a core part of their identity (this could also go the other way).