Sometimes you can get the feeling that everyone in a limited area gets a DNA match with everyone else. This can cause problems when you want to test whether a relationship is correct; it becomes difficult to distinguish interesting matches from uninteresting ones. It becomes difficult to know what you are actually seeing.
I am currently trying to confirm or dismiss a kinship from the Byske area and to get a better basis for assessment I wanted to estimate the presence of "random matches" or "false matches". How big a risk is it really that a person with roots in the area between Skellefteå and Piteå will be matched with any other person without there being any particular suspicion of kinship?
Completely unscientifically, I therefore asked in Facebook groups for Gedmatch numbers to people with main roots in the area. I received a set that I now compared on Gedmatch.
This is how pairwise total shared autosomal DNA and X-DNA look like in the group of 10 people from the area on Gedmatch. First with threshold 7 cM / 700 SNP:
Then with a slightly lower threshold, 5 cM / 500 SNPs:
Conclusion
Most people with roots in the Skellefteå - Piteå area have a clear autosomal DNA connection to each other. 67 % (2 of 3) in my random group have linkage that passes the 7 cM threshold and 82 % (5 of 6) pass the 5 cM threshold.
X-match is not as common but approximately 15 % (1 linkage in 6) have X-DNA that passes the 7 cM threshold and 35 % (1 linkage in 3) have X-DNA that passes the 5 cM threshold.
The composition of the group can be debated, but the tendency is clear. The majority match each other on the autosomal test and quite a few have X-match.
Jakob Norstedt
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