This comes from a religious study of several faiths which included Jehovah's Witnesses. There is a ton of data in this research and there are several things that are very interesting about JW's especially when compared with other denominations/faiths. You can do a basic search of the study here http://religions.pewforum.org/portraits
One thing that I was always curious about is how many born-ins leave the faith. Personally the vast majority of the born-ins that I knew left, including myself. So, I was not all that surprised to find that only 37% stay in, which happens to be the lowest retention rate of any of the studied religions/denominations. You can find that piece of data here, it is the next to last bullet point at the bottom of the page. http://religions.pewforum.org/reports/reports_1 which is also on pg 31 of the full report http://religions.pewforum.org/pdf/report-religious-landscape-study-full.pdf
For any researchers out there...I could not find there margin of error or if this was a statistically significant number. Oh well, still interesting :)