Barna doesn’t release how they conduct their studies — how they select the sample, etc. Don’t trust Barna — their findings cannot be verified.
Barna is in the business of selling books and DVDs that are solutions to the problems that their statistics outline.
I take their statistics with some disbelief as they do not mirror other publicly available statistics.
One of their more famous studies used the methodology of a very narrow definition of Bible believing Christian and came up with a lot of very negative results as well as a series of DVDs to cure the problem.
My wife who has studied Religion at the PHd level feels that Barna is doing a disservice to Christianity. I have to agree. I might also add that if they are not willing to expose their methodolgy and statistics to public scrutiny they are suspect.
I pay little attention to Barna now.