Comparing Match cMs At Different Sites

After a discussion at ISOGG Facebook I decided compare the data from matches who have results in multiple places including AncestryDNA, Family Tree DNA, 23andMe, and GEDmatch. I copied all my mother's match names from these site. I then sorted the names alphabetically. I found it was impossible to compare with AncestryDNA testers because most do not use their first and last names. Because so few testers use first and last names I was not able to use this method to find testers who were also in the other databases. It would be too time consuming to pick out those using their own first and last names. So I decided to do a more scaled down comparison using known cousins who have results in multiple places.

My results demonstrate that segment cM's are generally close to the same when comparing at Family Tree DNA, 23andMe, and GEDmatch. I did find a case where a segment cM's were 10 cM's apart between Family Tree DNA and GEDmatch. SNP totals at GEDmatch are often lower. Now I know to turn down the SNP totals when comparing at GEDmatch. I'll use 500 SNPs now.

Since AncestryDNA doesn't share their segment information I couldn't compare using segment totals. Instead I compared with cM totals. I didn't use segments under 7 cM's in the Family Tree DNA totals. It looks like GEDmatch always has the highest total cMs.  Ancestry always has the lowest. The average difference between Ancestry DNA and the other sites is 17 cM's. AncestryDNA phases and filters matches raw results, which is the reason for the differences in total cMs

Most of these matches are predicted in about the same cousin range at Ancestry and the other sites. The problem can be seen in my first chart. 23andMe, Family Tree DNA, and GEDmatch all show the person in chart one line 1 as a match. This person did test at Ancestry isn't a match with my mother there, even though she is a confirmed 4th cousin. I hadn't noticed until putting this together. I'm noticing more matches at the other sites who don't match at Ancestry. I have at least 5 confirmed cousins who did match at Ancestry, but don't now. Likely because of Timber. I'm not seeing this when looking at matches elsewhere. I'm sure some don't match at Family Tree DNA, but match elsewhere because of the 20 cM requirement. I have not encountered that because 1 cM segments are included.

Someone said if the results are different between sites what difference does it make? Ok, if each company has slightly different ranges but come up with the same matches then there isn't any problem. If confirmed cousin matches are being lost than I believe the companies should be rethinking their testing and matching procedures. Third cousins, and more distant cousins, are the ones affected by unreliable matching techniques. If a match shares only once segment they are more likely to be disappear as a match with additional processing.


Putting this together I have found more difficulties working with AncestryDNA than the others sites.

1. Ancestry doesn't allow you to download matches or their cM numbers (I used the chrome extension. Doesn't include cMs). 23andMe and Family Tree DNA allow you to download spreadsheets.
2. Ancestry should encourage testers to provide full names if they want to participate in sharing with other testers. I understand why some may not want to use their real names. They should use a consistent pseudonym, and use it everywhere, if they want to collaborate.
3. It would be nice if we could filter matches by total cMs.  
4. It would be nice if we could search by username.