Ancestry announced at a conference call today that there are some changes in the pipeline in terms of how our matches are reported. There will be three main changes:
1) Ancestry will provide a more accurate report on the number of segments shared with your matches. The updated matching algorithm may reduce the estimated number of segments you share with some. of your DNA matches. However, it won't change the estimated total amount of shared DNA (measured in centimorgans/cM) or the predicted relationship to your matches.
2) Ancestry will report the length of the largest shared segment. This is particularly important for people who are descended from endogamous populations. Knowing the length of the longest segment you and a DNA match have in common can help determine if you’re actually related. The longer the segment, the more likely you’re related. Segment length is also the easiest way to evaluate the difference between multiple matches that all show the same estimated relationship.
3) The matches will be re-calibrated to remove false matches so that the reported matches are more likely to be related through a recent common ancestor. Once the update is implemented, only matches which share 8 cM or more will be reported. Ancestry estimate that this will remove about two thirds of the false matches. All matches that fall below the new threshold will disappear from your match list with the exception of matches you have messaged, matches where you've added a note and matches you have added to a group by using the system of coloured dots. Starred matches will also be retained as they are considered part of a group. If you save a match below 8cM, your match will also have it saved without additional action needed. Any matches sharing less than 8 cM in total will no longer appear as common ancestor hints or in the ThruLines feature and this change may affect the number of ThruLines you see. If you want to save these matches you'll need to make sure you add them to one of your groups or add a note. Note that it is only the total cM shared after the application of the
Timber algorithm that is affected so you could still have matches which share some individual segments that are smaller than 8 cM so long as the sum total of all the segments is over 8 cM.
On site messaging will start to appear on the site in the next few days (this messaging is now live) to alert users to the updated matching system and a new matching white paper will be available later this week. (The White paper has now been published and can be accessed
here.) We can expect to see the new matching system rolled out in early August.
The increase in the match threshold will mean that many matches will disappear from our match lists. However, in practice, this is not going to have any effect on our genealogical research as these small matches have proved to be so unreliable that they are impossible to work with. The last time I analysed my matches at AncestryDNA and compared them with my parents' match lists I found that 54% of my matches in the 6-7 cM range did not match either of my parents and were therefore probably false positives.
(1) Clearly if there is over a 50% chance that a match will be false we cannot reliably assign these matches to a common ancestor, even if we can identify one in our shared family trees. Even if the match is real, the chances are still very low that it will be a reflection of a recent genealogical relationship and it is far more likely to be the result of very distant sharing.
(2)
I currently have over 32,000 matches at AncestryDNA which is far more than I can ever possibly cope with. However, if you really are desperate to go through your matches and check the 6 and 7 cm matches before they disappear you can use the filter under Shared DNA to set a custom cM range to identify these matches.
In other news AncestryDNA's corporate page has been updated to show that they have now tested
18 million people. AncestryDNA now have by far the largest genetic genealogy database in the world. 23andMe is the next largest with a database of
12 million people. MyHeritage have
4 million people in their database, while
FamilyTreeDNA have tested over two million people. (3)
The lockdown seems to have encouraged a renewed interest in family history so we can also look forward to receiving many more matches in the months and years to come.
Update 4th August
The roll out of the update has been delayed and will now be rolled out in stages. You will find full details, including FAQs, when you log into your AncestryDNA account.
Ancestry is now displaying decimal points for all matches sharing under 10 cM. All matches sharing under 8 cM will be removed at the end of August. This includes matches in the 7.5 to 7.9 cM range which were previously rounded up to 8 cM.
Further reading
Footnotes
1. See my blog post
Comparing parent and child matches at AncestryDNA from August 2017 for the full details of this analysis.
2. See the ISOGG Wiki page on
identity by descent which includes a chart from a 2015 paper by Doug Speed and David Balding providing the distribution of different-sized segments by generation.
3. FamilyTreeDNA do not publish details of the size of their autosomal DNA database. The two million figure about the number of people tested is taken from the FAQs on their home page. In the section headed "Who is FamilyTreeDNA?" they say: "Over 2 million people have tested with FamilyTreeDNA, resulting in the most comprehensive DNA matching database in the industry." FTDNA used to publish daily updates on the number of Y-DNA and mtDNA records in the database on their "
Why choose FamilyTreeDNA page?" However, the figures on this page have not been updated since July 2019. Martin McDowell did an analysis in February 2020 based on FTDNA kit numbers in which estimated that FTDNA's autosomal DNA database was approaching two million. See the blog post "
How big is the FamilyTreeDNA database" on the Genetic Genealogy Ireland website.
Updates
This page was updated on 15 July 2019 to include a third footnote to clarify information about the size of the FamilyTreeDNA database. It was updated on 16 July to include a link to the updated AncestryDNA white paper and a further reading list. It was also updated to clarify that starred matches will not be retained. The page was updated on 17 July to include a link to blog posts from Blaine Bettinger and Leah Larkin. The page was updated on 19 July following the receipt of an e-mail from AncestryDNA which clarified that starred matches would be retained after all and that any matches you save will also be automatically saved on your match's account. Additional information was added to the number points 1 and 2 with additional information from Ancestry about the changes in the reporting of segments. A link to Judy Russell's blog post was added on 28 July.