I have spent a significant amount of time and effort beating my head on brickwalls that appear when tracking Ancestry Autosomal DNA matches. I've had success in establishing document trails with matches as low as 7 CM and no success with other matches in the 20-25 cM range.
So I'm wondering just how much time (and bloody forehead) is warranted when trying to establish a actual cousin relationship at lower cM match levels.
I think that I already know that a certain percentage (an average) of actual cousin relationships will not be detected by autosomal DNA. See ISOGG Cousins statistics These I would call false-negatives.
What I don't know about is any study that give clues as to the percentage of detected autosomal DNA matches that are not an actual cousin relationship. I would refer to these as false-positives or error matches.
Consider the following: (all are Ancestry Autosomal DNA matches)
Helen and Nancy have a 11 cM match Helen and Sally Have a 21 cM match Nancy and Sally have a 1900 cM match
What is the likeihood that both (simultanously) the Helen/Sally and the Helen/Nancy matches are false-positives?
(By the way this is not theoretical, but rather an actual example in my family)