As you scroll down the Ancestry Match List page, you’ll eventually see a “Distant Cousin” marker. This is the Ancestry Distant Cousin Threshold. Matches above the mark are what Ancestry calls 4th-6th cousins (and closer). Matches below the mark are what they consider to be “Distant Cousins”.
The Ancestry Distant Cousin threshold is 20 centimorgans. In other words, a match is a distant cousin if they share less than 20 cM with you. But you may notice something odd about that in your own results.
Ever notice that the distant cousin threshold seems arbitrary?
Take a look at this pic of my Match List page. Katelyn is above the Distant Cousin marker and is shown as sharing 20 cM with me across 2 segments. But wait! Anthony is below the marker with the exact same numbers. Next down is J, who also shares 20 cM across 2 segments. And then there’s Jack at the top of the pic, with his 20 cM but a meagre 1 segment. How is he closer to me than Anthony and j?
The answer is that Ancestry calculates shared centimorgans to at least one decimal place but only displays a rounded number. Below-threshold Anthony actually shares 19.9 cM with me, and the display is rounding up to 20. Above-threshold Katelyn actually shares 20.1 cM with me, and the display is rounding down to 20.
This is clearly seen in my personal Match Spreadsheet:
Was it ever thus?
I can’t remember if Ancestry has changed the way it shows centimorgans. My memory is that the decimal number used to be accessible somewhere from the main display, but I can’t find it anymore. Drop me a comment if you can spot it.
Thankfully, the full data is available via the Boyne Analytics spreadsheets. Give them a try, and harness the full power of your Ancestry data.