Hi, HS!
My 99.9% statistic stems from local experience and what I hear various traveling overseers talk about.
In our congregation there is not a single youth over the past decade that has completely avoided pressures to act different than they learn from WTS literature. This is what I meant. This includes everything from, pretending to "go in service" (movies instead), to studying playboy instead of The Watchtower, to driving under the influence, to fornication, to suicide.
The best example we could muster right now would be a young man getting ready to go to Bethel who only a few years ago was… well you know, with an old copy of… against the wall… Our PO almost had a fist… I meant a fit. When his parents made him confess, there turned out to be more to the story than any solo action.
As for how the figures Maximus asserted are found out, as you say, the WTS has COs doing all sorts of surveys, all the time. This is just one of them.