I’m just thinking outloud and, of course, you are welcome to think aloud with me.
(1) There are the metrics used in the Yearbook totals. The average publisher count measures only those who reported time. Likely, most of those who attend the Memorial are those inactive or somehow associated in some way, shape, or form.
(2) There is meeting attendance. Unfortunately, only HQ knows this count for sure.
(3) There are donations. Only HQ knows this. This might be a useful barometer: since donations are relatively anonymous, while people may feel compelled to attend meetings and knock on doors, how much they contribute could indicate apathy or doubt. Of course, it can also be dependent on the economy.
(4) The number of elders and ministerial servants. Only HQ knows, but by all accounts this number has been in freefall for the last two decades. Men are not “stepping up to the plate.”
There are various ways of measuring this group. Unfortunately, since we do not have all the information, much of it is simply educated conjecture.