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Dennis ,

You pointed a very timely observation of the the reports for a correct
interpretation of the data.
Good deed .

Regards,

Dennis Greene wrote:

> 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

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