Could it be that a group of matches with the largest cM value in the mid 20s are out-of-range?

2018-04-23 20:08:44

WARNING: This is another question that involves endogamous genetic genealogy. (Not applicable to "normal" or "standard" DNA evaluation).

I am still struggling to find some statistical limits/boundaries for evaluating my endogamous ancestry. Maybe I need to read more on the subject... but this subject matter is fairly limited -- and usually complex for me, at the best of times.

In my question I am wondering if my group of atDNA matches, that have a "largest cM" value of about 25, might have the common ancestor that is out of range (or pre-1700s/ pre-source). Part of the problem is, that I have nearly ten people in this group with a value of >20 cM, and their family tree's are well developed, yet not even three of these matches share one common ancestor!

I'm wondering what the outer limits are, in terms of cM, for endogamous groups. I thought that >20 cM would surely be within reach... but maybe not. And are there other ways of evaluating this situation...