The plural of anecdote is not data.
From the JREF contest:
randi.org/site/index.php/1m-challenge/challenge-faq.html
Religion has a long history of ascribing “divine punishment” or “demonic activity” to illnesses with entirely natural causes. As far as I know, exorcism has never been meaningfully tested as a cure for anything.
I would be willing to participate in a study where people deliberately got themselves possessed so that the effects of possession could be verified. That would go hand in hand with testing what methods of possession work (e.g. do ouija boards produce the same symptoms?) After the symptoms and causes were established, we could test whether or not exorcism were effective cures for the symptoms of possession.