They might be above .90 in some instances. With scanty evidence the probability would be lowered to far below .90. There is one consolation, however. The probability that any random individual is your ancestor increases with time. It is almost a certainty that William the Conqueror is your ancestor if you have much English blood.
Genealogy becomes the history of a race after a few centuries.
There are many hidden assumptions in genealogical work. These assumptions account for much of the uncertainty which arises. If any form of inference, such as a logical deduction, is used as proof, the validity of the assumptions as well as the validity of each step in the deduction must be considered. The following example is given so that the reader can look for the hidden assumptions and can better understand the nature of proof in genealogy. John Williams was the father of Elizabeth Williams.