> Studies have been done with a lot more statistical rigor
> than I can remember or recite here, but they usually boil
> down to this:
>
> If you take the speed limit signs off of a road,
> eventually the average rate of speed on that road
> will settle out to 10-20 MPH over the previously posted
> limit. In this case, "average" is the 85th percentile
> speed, which is the mathematical average speed that the
> "middle" 85% of the drivers travel at.
In geophysics -- a science whose data is frequently
contaminated with noise -- we frequently use what is called
an 'alpha trim mean', especially in data scaling. In that case,
the 'alpha' parameter is similar to the 15% number you quote.
In our case, using your numbers, the top 15% and the bottom
15% of the observations are not representative of realistic data;
Therefore, they are 'trimmed off before the arithmatic mean is
found. This explanation probably helps no one. Sorry.
rick
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