Never really thought about it before.
One way that occurs to me (if JWs were not so secretive and paranoid) would be to somehow construct a large enough population of JWs to be able to form a random sample. Then you could poll that random sample, and extrapolate results.
However, given that being paranoid and secretive and not ever daring to give your true opinion, if it happens to conflict with official doctrine, goes hand-in-glove with being a JW, that is not a realistic method.
Given the paranoia of voicing any deep concern to any "insider", and the near-manaical belief that the image one must present to "outsiders" is one of a blissful "spiritual paradise", getting straight answers to any question would be exceedingly difficult for anyone.
Perhaps if there were enough insider "secret apostates" in enough locations within the congregations, and their efforts were coordinated, enough observations could be done to summarize something. You would need to coordinate well--what information to look for, how to go about it, making sure a good cross-section of JW demographics was covered, etc.
This idea has occurred to me: If the Society were "smart" (something I don't foresee happening any time soon), they would proclaim some sort of "general amnesty day". They would encourage any and all to voice whatever fears, concerns, quibbles, whatever, on that day, with no fear of repercussions. Then, with that more or less accurate barometer of where their deepest problems are, they could go about fixing them.
But I don't think the Society really wants to know. The cynical side of me says they don't even care.