Some time ago, maybe a year or two, two men that I knew of did the test for their DNA under the Johnson-Johnston Family Tree DNA project. It turned out that they were a perfect match.
These two Johnstons were from different parts of the US, one of them being New Jersey, and I think he said Pennsylvania roots. This proves only that somewhere back there in the last fourteen generations they had a mutual ancestor. The other was from Alabama-Georgia.
More Johnsons and some Johnstons took the test, but were from three to five or seven numbers off. I found that a good many of the test takers had the same first five to seven numbers in identical order. Guess this was the original group of peoples who went to make up the Scots. Ours seems to have included some Nordic inheritance.
To have any kind of conclusions to draw from this study, many more Johnstons would have to take it, then compare results.
You wrote, "To have any kind of conclusions to draw from this study, many more Johnstons would have to take it, then compare results."
That's not exactly true but it does depend on what conclusion you want to draw. One thing the study has proved over and over is that not all people with the surname are related. In some of the people tested it is pretty certain they have not had the same male ancestor for thousands of years.
DNA is very good at proving negetive relationships.
DNA is frequently used to prove that a certain person did not commit a crime. That is on the news every night.
However, a match between two DNA samples is not absolute proof that the samples are from the same person. That means it is stated as a probability, not a certainty. Anybody who does a paternity test gets a probability of paternity, like 1 in 13,000,000 or something.
Genealogy testing is not providing proof of a relationship. It is just another piece of evidence.
When you find a name you are looking for on a census that is not proof you have found the person you are looking for. It could be another person with the same name. However if it is the right county and the wife and kids names and ages match what you expect you can be pretty confident you have found the right person. Unfortunately you have not proved it. That's the way it is with DNA. It's just another fact you look at.
I am certainly not claiming to be an expert on DNA. However, when it comes to genealogy, perhaps no expert, but I was a Certified Genealogist (from Board of Certification of Genealogists) for twenty years, and once a member of that Board.
The important thing to remember is that results from a DNA test is just another piece of evidence. I think it is exciting in the limited cases where it works but it will never replace old-fashoned genealogy.
A DNA test without the written records is pretty meaningless. As a genealogist I want to know about my ancestors; where they lived, who they married, what they did for a living, and where they are buried. That comes from paper and microfilm.
On the other hand, this computer is very handy for sharing that written information with others. My computer is another tool I use.
And a DNA test can prove that two men are not related. That can be important in some cases. It is a tool that gives the right answer if the right question was asked.
DNA results only traces one line, the direct male line.
At 10 generations a person has 1,024 ancestors, half of them male. DNA can tell you about one of those men -- but not very much about him.