Ancestry.com has a feature where it will use all available trees to find a common ancestor for a DNA match. I believe that it looks for overlaps in trees. It has done a good job for many of my matches.
There are a few common ancestors that it has not found, even though I copied many people from the match's tree. There should be plenty of overlap in our trees for Ancestry to find the common ancestor.
Does anyone know exactly what the algorithm looks for?
I know that the task runs in the background, sometimes the paths to the common ancestor won't show up for a few days. It has been a lot more than a few days for the cases that I am referring to.
Edit: more background. Here are snippets from the two trees. Yesterday, only the people in red were common to both trees. In my experience this should be enough. I just added the siblings of Helen Beg (b1899) to my tree, we will see if this makes a difference.
The algorithm is very impressive in some other cases. For one case, it found the common ancestor when the other person had a poor tree, and my tree didn't connect. It used a third tree to connect everything!