That sounds to me like a reference to the GEE, although it is ambiguous. (Some of the threads it's used on support this interpretation.)
I think this tag's name will inevitably cause it to be used incorrectly by the many statistical novices who ask questions here. I don't immediately see how that can be addressed effectively. For example, it has an excerpt, but many people don't read them anyway. Thus, I don't think making it a synonym of something else is the right option. (We'll just end up finding some other tag commonly misapplied.)
I think the best we can do is probably just to remove the tag from threads and let the SE system purge it after 24 hours. An alternative would be to replace it with a new tag that has a different name that won't be misunderstood by novices. I'm not sure what the appropriate name would be, though, and existing tags, such as gee, might be enough already. The tag may have been created by @JiebiaoWang; it would be worth seeing what he has to say. I attempted to leave him a comment alerting him to this thread.
Update: It seems
[marginal-model] might be a good name for a new tag to put in place of the old one (h/t @JakeWestfall). I propose that we work through the threads currently tagged with population-average and, where appropriate, replace it with the new tag. Where the tag had been misapplied, it can just be removed. Once it doesn't exist on any threads, the SE system will automatically purge it in 24 hours.
Update 2: Given the response to this, I will go through the threads tagged with
[population-average] and remove the tag. I may replace it with
[gee] (or something else) as seems most appropriate. I have now done the first such thread (here).
Update 3: I have now removed or replaced the tag on all threads. The tag should disappear after a day or so.