Clusters form on a Common Ancestor (CA). We don’t have proof of this but a) it makes sense (why else would our Matches match each other in a Cluster?); and b) it sure seems to work (I’ve found many new CAs with Matches, just by focusing on the CA in a Cluster).
So, with this concept in mind, let’s use our Clusters!
- Known CA – If you *know*, or even suspect, the CA of a Cluster, search other Matches in that Cluster for that CA or location or a collateral line. If a Cluster Match has a good Tree, there’s a good chance you’ll find the CA in their Tree. There’s a good chance multiple Matches in a Cluster will all have the same CA. Armed with a known CA, I’ve often been able to build out a Match’s Tree to that CA.
- Unknown CA – If you don’t have a clue to the Cluster CA, find the most likely CA among the Matches – whether you have that surname or not. Let the Matches tell you the Cluster CA – per this blogpost. This is also effective for Brick Walls and unknown parentage.
- Suspect CA – If some on the internet propose an Ancestor for one of your lines without proof, or if you are suspicious of their “proof”, test out that Ancestor. Look for that surname among the Matches in appropriate Clusters. “Appropriate” means these Clusters are probably on that line. Try the Unknown CA process and see if this same surname comes up. Clearly, if many people have bought into this Suspect CA, this process won’t work (however, then using this process with the Suspect CA’s mother’s surname, may be helpful). Example: During 40 years of research on my NEWLON line, many had heard the claim by one researcher that a spouse was “Martha JANNEY”, but without proof, few used that information. So I decided to test it. Virtually none of my Cluster Matches had the JANNEY surname; but many had the CUMMIN/GS surname. In fact, searching all of my DNA Matches (over 125,000 of them) turned up 17 Matches (down to 6cM) with the JANNEY surname in Loudoun Co, VA – none in any of my “appropriate” Clusters.
Bottom line: Use the concept that Clusters form on a CA. Use it to find CAs with more Matches; Use it to break through Brick Walls or explore Clusters without a CA. Use it to *test* likely or suspicious surnames in selected Clusters – if the CA is correct, it should show up in multiple Matches in a Cluster.
[19J] Segment-ology: Use Clusters! by Jim Bartlett 20200705
segment-ology 
Jim
I just got back into working with Gedmatch as I have been working on all of my Thrulines on Ancestry. My focus now is on my 23andme matches. I have my Montenegrin cousins 23andme profiles on Gedmatch. This my first look at the new tier1 tools on Gedmatch and found the Cluster tool (Beta). I am a triangulation guy and have not done much with clustering due to the time involved. After sever trial runs experimenting with the tool I was able to blend the two ideas in to something that will save me hours of manual work. By running the clustering tool and then submitting the resulting clusters to the Triangulation tool Gedmatch created a Triangulation report that gives me a working document of matches that are of vastly improved data and a size I can manage.
Doug
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Jim,
I was fortunate enough to do the Ancestry downloads necessary to get my full cluster data package via the Brecher cluster too before Ancestry shut the Brecher tool down. This Brecher cluster data has been extremely useful to me so I have been talking it up with other genealogists, but now they can’t get the kind of data that I got. You mentioned in your previous post that the Ancestry clustering tools were “in a pause mode.” Do you have any good predictions about how we might get the Brecher capability back.
Thanks for all of your good posts.
James Marion Baker, CG
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James, I have no insider information. Being able to capture all your data is a valuable tool, and I HOPE AncestryDNA will allow it after they reconfigure everything. I wouldn’t hurt to let Ancestry know you want this ability. Jim
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Paul, Thanks for your input and encouragement. On one level DNA is easy – we get it from our Ancestors. On the other hand there is always more to figure out. In genealogy, we’ve said: when you solve a problem (find an Ancestor), it creates two new problems (the parents of that Ancestor). This works in our favor in genetic genealogy – as we link an Ancestor to a DNA segment, we limit the possibilities for the next generation back for that segment to the the parents of that Ancestor.
I think we Segmentologists can navigate genetic genealogy a little better.
Jim
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Jim, I completely agree and I love the math and science of it all. It is like a never ending jigsaw puzzle. Paul
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A great article. I am a believer. All DNA comes from only 1 of 2 places…your mother or your father. We just need to figure out which person and our CA for all matches.
This week on FTDNA I reexamined my X DNA matches and found a 19 cM segment match with no tree. Because of unique inheritance patterns, I was quickly able to narrow our CA at the 4 great grandmother location.
I am a believer in segmentology.
Paul Baltzer
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