Several recent postings have mentioned that 57 is considered the median age
for the onset of PD. As far as I can see most have assumed that "median"
and "average" mean the same thing and have concluded from this that half of
the PD population are under the age of 57. However median and average are
not the same thing, nor can you use either of them to conclude that half the
PD population are under 57.
An average age for something to happen tells you at what age the event is
most likely to happen. In our case it would be calculated by adding up the
age at onset of every PWP, and then dividing the total by the number of PWP
(several million bits of information processed). Even if 57 were the
average age, it would mean that 57 was the age at which most people
experienced onset and consequently most PWP are older than 57.
Median refers to the age itself, not to the numbers afflicted. It is
derived by finding the middle point between the age of the youngest and
oldest age of onset (two bits of information processed). To have any meaning
at all, a median value would need to be quoted in its context. By itself
the statement "the median age of onset is 57" tells us very little. It
would be true in both the following cases:
1. Youngest age of onset = 50
Oldest age of onset = 64
Median age of onset = 57
2. Youngest age of onset = 20
Oldest age of onset = 94
Median age of onset = 57
Dennis
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Dennis Greene 48/11
"It is better to be a crystal and be broken,
Than to remain perfect like a tile upon the housetop."
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http://members.networx.net.au/~dennisg/
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