Friday, November 7, 2014
GEDmatch Segment Size May Be Different From That Reported by Your Testing Company
I found a long awaited match on our Owens line at Family Tree DNA yesterday. This person evidently transferred her results to Family Tree DNA immediately after this service was announced in October. I copied her email address and went to GEDmatch to see if she appeared in my Aunts results. I found her listed with two kits. I compared her two kits. Both kits showed identical results with my Aunt. Compared against my Aunt at GEDmatch I noticed their longest shared segment was 14.7. Her longest segment with my Aunt at Family Tree DNA was reported as 8.68?
A basically 6 cM difference. I've emailed this match to be sure we're comparing apples to apples. If there really is a 6 cM difference for the result on 19 (see their charts below) then there is a big problem somewhere?
Comparing results from Family Tree DNA, 23andMe, and GEDmatch there is a consistent 1 cM difference at GEDmatch with the occasional 3 cM difference. I checked to see if 23andMe was any closer to GEDmatch, but they also differ by 1 to 3 cMs (see above). I don't know how well 23andMe results compare with Family Tree DNA outside GEDmatch?
If we're using cM's to determine our relationship to a match and to triangulate it's important to have consistent numbers. Hope we can eventually compare with equivalent, agreed upon, standards. Right now I don't know whether I should accept the 14.7 result as correct or the 8.68 result? Or are we looking at different kits? Hoping to get answer soon?