As we go further back in time, the odds improve that any given individual living then will be an ancestor (assuming that the individual did leave some descendants). Let's look at another question, namely, what are the odds that an individual ten generations in back of us is really our ancestor. For example, what is the chance that Henry Krankenhaus is really my ancestor. Let's just say that he is ten generations away from me. I have some data about each generation which may or may not be true. At each step in the line going back to him, I will say the data has a probability of about 0.9 of being true. A probability of 1.0 leaves absolutely no room for doubt. A probability of 0.0 is a complete lack of knowledge. So at an assessment of 0.9, I am saying that the data looks good but there is an allowance for a mistake.
Going back ten generations with a certainty of 0.9 at each step, the odds that a person that far removed is really an ancestor are only 0.35. Or stated in another way, the odds are only about one in three that he or she is truly an ancestor. So as people start collecting ancestors, which is becoming a fad today, are they collecting facts or are they collecting junk? It does not take much before the limbs become trash.
Errors arise in several ways. A lack of knowledge leads to suppositions, which become embedded as facts. I am surprised at how many trees come my way in which John Wilhoit married Margaret Weaver, the granddaughter of Peter Weaver. Suppositions, not founded on proven fact, must be recognized. They may be valuable as leads for research but the danger is that the supposition is quoted without the qualifications and limitations. If there is a lack of knowledge, it is best to leave the line open than to state what is at best a guess.
In a recent note, I quoted the situation of Catherine who had a daughter whom she admitted was not her husband's child. Not all people are as honest as Catherine. A women on the web told her story of research which started with the generations just in back of her. An unwed teenager was married to cover up the birth of a child by a man other than the one she married. This same woman told also of finding that a birth certificate was false in the previous generation. The true father was the priest but the birth certificate said something else. Or a child may be adopted or reared in another family without the child being told. (Or the hospital mixed up children.)
At each fork in the tree, one must attach some probability that the facts are indeed true. Then to find the probability that the line from here to there is really true is the product (as in multiplication) of the individual probabilities. As quoted above, a probability of 0.9 at each step, which to most people is almost a certainty, leads at the tenth generation to a probability of 0.35 on the path being correct. That is less than the odds of a heads on a coin toss. Increase the odds to 0.95 at each step and the probability of the path being correct is 0.6 which is only slightly better than tossing a coin.
We gratefully acknowledge the work of John Blankenbaker who published over 2,500 Germanna History Notes via the Germanna-L@rootsweb.com email list from 1997 to 2008. We are equally thankful to George Durman (Sgt. George) for hosting the list and republishing the notes via rootsweb.com.