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Re: Cops vs.LBCs

To: "David Breneman" <idcb@airborne.com>, <mgs@autox.team.net>
Subject: Re: Cops vs.LBCs
From: "Rick Lindsay" <ROLindsay@Emeraldgrc.com>
Date: Tue, 20 Aug 2002 14:42:29 -0500
> 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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