Dick Eastman posted a response to the current fascination with finding family relationships between feuding american politicians. I cannot imagine too many Australians being all that interested to learn that Julia Gillard and Tony Abbot were sixth cousins, but you never know.
The aspect of Dick's article that did interest me was that in order to do some back of the envelope calculations, he wrote Let's assume that there is a new generation born every twenty-five years.
I have no dispute with any of the arithmetic and have used the same approximation myself in the past. It is almost certainly valid when dealing with whole populations, but how well does it apply to a single family (namely, ours)?
The mean value for the birthdates of our direct ancestors (and of a descendent) are set out below.
| Generation of | Mean birth year | Duration |
|---|---|---|
| 3xgreat grandparents | 1837 | 28 years |
| 2xgreat grandparents | 1875 | 30 years |
| Great grandparents | 1905 | 25 years |
| Grandparents | 1930 | 25 years |
| Our parents | 1955 | 26 years |
| Ourselves | 1981 | 27 years |
| Our children | 2008 |
Which might appear to be a very long way of saying "Guess what, Dick Eastman was exactly right in his working assumption and you just wasted an hour." But as always the important learning lies in the working out rather than the answer.
Mean values are a very limited property of a set of numbers. They do not reveal that the range of ages within a generation can be as much as the duration of the generation. Should we speak of a group of ancestors as "a generation" when the range of their births and deaths is so great that there life experiences must have been very different?
In particular, what does it mean that two of our 3xgreat grandparents were born after one of our 2xgreat grandparents. That is a tale for another time.
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