I'm continuing with more material from Ralph Taylor on the meaning of matches. What does it mean if you match 67 out of 67, commonly written 67/67, with another person? There is about a 50% likelihood that you share the same great-grandfather! Thus, you are second cousins. You could be slightly closer related or slightly more distantly related, but you are related recently as genealogists measure relationship!
As Ralph Taylor says, "67-marker matches, if you’re lucky enough to find one, are very precise; you may need to account for the birth dates of the matching individuals. The main problem is that most people have done a lesser test, so you can only compare against the markers they had tested." Thus, refer to earlier blogs which discuss 37 marker matches and fewer matches.
Should you match in the range of 65-67/67: You share a common ancestor and he almost certainly lived within a genealogic time frame.
o A perfect 67/67: The 90% probability level is a TMRCA of <= 11 generations, >= 1735 AD; 70% is <= 6 gen., >= 1860 AD; & it’s 50% probable that your common ancestor is your great-great-grandfather.
o 66/67: 90% is <= 18 gen., >= 1560 AD; 70% is <= 18 gen., >= 1730 AD; 50% is <= 7 gen., 1835 AD.
o 65/67: 90% is <= 24 gen., >= 1410 AD; 70% is <= 16 gen., >= 1610; 50% is <= 12 gen., >= 1710 AD.
Should you match in the range of 62-64/67: You share a common ancestor, who may or may not have lived in a genealogic time frame.
o 64/67: 90% is <= 31 gen., >= 1235; 70% is <= 22 gen., >= 1460; 50% is <= 16 gen., 1610.
o 63/67: 90% is <= 37 gen., >= 1085 AD; 70% is <= 27 gen., >= 1330 AD; 50% is <= 21 gen., >= 1480 AD.
o 62/67: 90% is <= 43 gen., >= 930 AD; 70% is <= 32 gen., >= 1200 AD; 50% is <= 26 gen., >= 1360 AD.
We've have spent a number of blog entries discussing the statistical meaning of matches. You do not need to completely understand statistics or probability, though. What you do need to do is to investigate the family trees of anyone who closely matches you. For example, with a "perfect" match of 67/67, you and the other person should be able to find your common ancestor fairly easily. One hopes that each of you has some information about your mutual family tree so that you each gain significant new information. After all, that's why you chose to participate in this project.
The good news is that eventually sufficient y-DNA records and pedigrees will exist to satisfy every participant's desire for knowledge of their roots!
Best wishes as we move towards that goal.