Where as you can’t speculate on the date of the Norman invasion, that is an established ‘fact’ you either you know it or you don’t. Any clumping in the data in regard to the Norman Conquest survey is probably due more to people guessing against their partially remembered knowledge of history lessons.
The reason that I mentioned the Norman Invasion is that it is a known. Before placing any reliance on projections of something in the future it's good to have some confirmation that the method actually works. Tests can only be run against a known result, sorta a standard. It's like testing radiometric dating against something of known age.
The amazing thing about the Delphi Method is that when a statistically large sample is used and questions asked where the respondents cannot know the answer, there really does seem to be some correlation between the responses and fact. The problem when we consider applying it to religion is that frankly, it's unlikely we will ever be able to arrive at a conclusion that can be transfered to others still living.
If though, the method really works, then there is the possibility that similar value might be placed on such questions even though we will never know the answer for sure.
Aslan is not a Tame Lion