I recently came across the claim that 80-90% of people who convert to Christianity will eventually revert to their former beliefs/way of life. While I can't speak to the world at large, my experience is that this number is closer to 40%. I was wondering if there were some actual numbers one could look up to confirm this. I was also wondering if there were a way to somehow subtract the number of people marrying into the faith (as that accounts for a number of baptisms/confirmations too).
It would appear that the answer to your question that is reliable, and applicable across the board, is a resounding "no". Within denominational or individual Church/affiliation boundaries, there are plenty, but the numbers vary
Explanation of the short answer:
I've found several sources that give "fall away rates" with varying degrees of source citations. From http://www.frontlinemin.org/decisionism.asp
(and I'm not sure if these numbers are trustworthy - a number of them are vague and dont' give the Church, Crusade, denomination, at all. They could easily be fabricated.)
But another source gives different statistics, way off from the ones above.
(and still, I wonder where they get their numbers.)
As for the anecdotal evidence supplied from the teaching you linked to in your question, Mr. Comfort gives the source for that particular statistic as a copy of American Horizons Magazine, the official magazine of the Church in question. (Coincidentally, the last one in my first bulleted list...)
But as for an official, scientific study, or a survey that follows real methods, I find no evidence. It would appear that the numbers are based on anecdotal evidence, or as Mr. Comfort states. "Church records" - records shared from Pastors at Churches. Even in the Hell's Best Kept Secret teaching, he states that the number is based on Church records that he has had access to. He never claims that this is a universally applicable statistic.
This is, at least, feasible. I know that in our Church, we keep records of those who have prayed the sinner's prayer, and we have records of who is still in attendance, and who has left, and for what reason. (Church discipline, asked to be removed from membership, etc.) I'm sure if I talked to my Pastor, I could come up with a calculation for our own Church's "fall away rate".
But is that indicative of the fall-away rate for Christianity as a whole? no.
And given the fractured nature of Christianity, and even the difficulty in defining what "falling away" really means, I'd say it's not feasible to get such results. How would you define "falling away"? No longer attending Church? What about those that don't attend a Church at all, but are active in prayer, evangelism, and just don't want to go to a local Church? If not based on active Church membership, then what?