Here's an indication of the staggering difference in results using the two methods:
Using the common approach of allowing people to self-identify as evangelicals, 40% of adults classify themselves as such. Among them, 83% are likely to vote in November. Among the self-reported evangelicals who are likely to vote, John McCain holds a narrow 39% to 37% lead over Sen. Obama. Nearly one-quarter of this segment (23%) is still undecided about who they will vote for.Wow. Those are some crazy wild disparities. You can find the full survey results here.
Using the Barna approach of studying people’s core religious beliefs produces a very different outcome. Just 8% of the adult population qualifies as evangelical based on their answers to the nine belief questions. Among that segment, a significantly higher proportion (90%) is likely to vote in November, and Sen. McCain holds a huge lead (61%-17%) over the Democratic nominee. Overall, just 14% of this group remains undecided regarding their candidate of choice.