Shared cM histograms: did you know? #3

The third post in a new series aiming to surface features at DNA Painter that you might not be aware of. This time it’s Shared cM histograms…

The Shared cM Project tool is a popular interactive tool that allows you to enter an amount of shared DNA and explore relationship possibilities. The tool uses data from the Shared cM Project, a collection of crowd-sourced data that has been collated by genealogist Blaine Bettinger since 2015.

A screenshot of the Shared cM Project Tool

I recently had the pleasure of meeting lots of DNA Painter users at the RootsTech conference in Salt Lake City. Many people told me how much they appreciated the shared cM tool. I always asked them, “Do you also use the histograms?” In almost every case, it turned out that the user had no idea what I was talking about. So, here’s what I was talking about!

A recap on how the tool works

Near the top, you’ll see a box, ‘Enter the total number of cM for your match.’

The box at the top of the Shared cM project tool where you enter the number of cMs shared

After you enter the number of centimorgans (cMs) of DNA shared, two things happen:

The grid of possible relationships is filtered so that just the relationships that are possible for the entered amount are highlighted
A set of relationship probabilities are output above the grid of relationships

Screenshots showing the filtered grid of relationships and the probabilities shown after 543cM is entered into the tool

Shared cM Project Tool histograms

At this point, many users move onto their next task, but if you stay, you can explore possible relationships more closely:

Click on any relationship (this could be a relationship name, or a relationship box in the grid), to view a histogram
This is a simple graph that can give you a more detailed sense of how the amount that you share with your match compares with the amounts that other genealogists share with people with whom they share this relationship.

An example

As you can see in the screenshot above, there are quite a few possibilities for a match of 543cM.

Clearly the first thing to do is look at shared matches and investigate this match’s family history. You might then come back with a hypothesis based on ages and locations.

Let’s say that based on our research, we think this match could be a half aunt. In order to see more detail, we can view the histogram for this relationship by clicking on the text ‘Half Aunt/Uncle’ or on the Half Aunt/Uncle relationship box in the grid.

The relationship boxes and links you can click on to see a shared cM histogram for Half Aunt / Uncle — The relationship boxes and links you can click on to see a histogram for Half Aunt / Uncle

What the shared cM histograms provide

This immediately provides a lot of additional information.

The shared cM histogram for the relationship Half Aunt / Uncle with an arrow indicating the position of 543cM, the amount entered. — The histogram for the relationship Half Aunt / Uncle with an arrow indicating the position of 543cM, the amount entered.

The screenshot above shows the distribution of submissions to the Shared cM Project for the relationship ‘Half Aunt / Uncle.’

We can see that while 543cM is possible for this relationship, over 97% of reported cases shared more than this. It would therefore make sense to look for additional evidence to support this hypothesis.

The information provided is as follows:

The numbers along the X-axis represent each ‘bin’ of submissions
For example, 600 represents all the submissions for ‘Half Aunt / Uncle’ that were between 501 and 600cM
The numbers above the blue blocks are the number of submissions for that ‘bin’
A red arrow indicates where the number that you’ve entered fits in alongside the other submissions

Beneath the graph you’ll see two horizontal arrows:

One shows the approximate percentage of submissions that were less than the amount you entered (in this case, 2.6%)
The other shows the approximate percentage that were more than the amount entered (in this case, 97.4%)

If you’re using the version of the tool that accepts two values, you’ll see a red arrow for both the amounts entered. For more information about this version, please read Blaine Bettinger’s blog post.

Additional information in the histogram overlay

Beneath the graph, the explanatory text includes data on how many submissions there were in total alongside the mean value and the standard deviation.

The additional information below the shared cM histogram

Finally

I hope you find the shared cM histograms useful. If so, please spread the word… After the conference, I added a small graphic to try and surface this feature more:

I hope you are now fully aware and find it useful!