Good question, and one I have been asked several times now! I was shown an article once that talked about special needs families as being the most unchurched group in our nation. Naturally, I couldn't remember where I'd seen it so I did a little google search to try and find it. I found the article I eventually linked in my blog post and it was that article which stated over 90% of families with special needs children do not attend church. I had intended to go back and see if I could track down the original source of that statistic but forgot to do so.
In the four days (!) since I published that post, I have been in contact with a pastor who is writing his dissertation on the subject of integrating people with special needs into the local church. I asked him if he had found the original source of that statistic and basically he said it is very elusive--he's been looking for it for years. I have tried to google the information several different ways but all I get are articles much like the first one: people quote a number but don't provide a source (much like yours truly did).
I saw several different numbers, too. I used 90% in my post because that is the first number I saw. Others say it is 80%, others say it is 95%. Joni Eareckson Tada's website resources refer to people affected by disabilities as one of the largest unreached people groups today.
All that to say that I still don't know who/what organization derived these numbers or if it is a case of one person guessing a number, publishing the number, then seeing it repeated because the actual statistic doesn't exist.
This I do know: if you've read the often raw comments to my blog post or on facebook or even other websites (I ran into a full-fledged discussion of my post while looking for the stat this morning), you can easily surmise that it's a problem worth probing and discussing. Families are hurting; some are turning away from the church completely because they don't see it has any value in the life of their special child. While I strongly believe that Christ has ultimate and eternal value for all of us, I am also strongly interested in looking deeply into the reasons the local church so frequently fails to exemplify those values. (Yes, I know, huge issue, very deep, same can be said of any value the Bible espouses and we fall short. I still think it worthy of consideration.)
Thank you for asking so nicely.