HFA yDNA Project
Math terms used in this web site
What are Minimum, Mode, Median, Mean, and Maximum?
Understanding that marker matches/mismatches need to be understood statistically.
Assume we gathered a group that consists of all of the male decendants of a common ancestor who lived 8 generations ago. Assume that there were 500 of these male descendants. Forget how we could know that, just accept it for now. If we measured the standard panel of 37 yDNA markers for all of them and compared it against those same markers from a sample that we had of that common ancestor, this is what we would likely find:
The majority of participants would have the same markers as their ancestor. Many, but fewer would have 36 of the 37 match. Many, but even fewer, would match on 35 of 37 markers. The number of participants with even more mismatches will decrease quickly. We might even find a participant who mismatched by six or eight markers. We might find that 60% of the participants had perfect matches; that 80% had 0 or 1 mismatch; that 95% had 0,1, or 2 mismatches; that 98.5% had 0, 1, 2, or 3 mismatches; that 99.99% had 0, 1, 2, 3, or 4 mismatches. But there might be one participant who had 8 mismatches. His numbers would be called an "outlier". Typically, we would say that 6 or more mismatches indicate that participants don't share a common ancestor. However, remember the possibility of outliers, when you are comparing mismatches of 6-10.