Re: Distribution of a Percetile

"Bob Wheeler" <bwheeler@xxxxxxxxx> wrote in message
news:579f3$4331ae79$466eb713$12525@xxxxxxxxxxxxxxxxxxxxxxx
> Stephen Clark wrote:
>> I have some traffic speed data which consists of the speeds of individual
>> vehicles as they pass a point on the road. These daily speed measurements
>> (the sample size, n, is large but variable each day) can be charted and
>> approximated with a skewed distribution.
>>
>> Some data was collected before and some after a change. I would like to
>> obtain the distribution of the 85th%ile of these speed distributions and
>> test whether they are the same.
>>
>> Can someone point me to a web resource which will allow me to conduct
>> this test? Thanks.
>>
>>
> You would be better advised to assume a distribution, likely exponential,
> and test estimates of the parameters. I'd use the mean. By testing a
> percentile you are throwing away information.
>
OK, but the "intervention" was designed to reduce the number of speeding
vehicles which is why I am interested in concentrating on the impact at the
upper tail of the speed distribution. It is possible that the intervention
worked but the mean did not change significantly.



.

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