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Re: compute quartiles of a distribution [message #77911 is a reply to message #77909]

Tue, 18 October 2011 12:48

Jeremy Bailin
Messages: 618
Registered: April 2008

Senior Member

On 10/18/11 12:12 PM, bing999 wrote:
> Thanks to both of you for your answers.
>
> The procedures in summary.pro and cgBoxPlot.pro compute "real"
> quartiles. Actually, I should not have used this word in my case i
> guess.
>
> What I want is the interval [M-Q;M+Q] which encompass 75% of the
> values of the sample around the mean (not the median) value M, where Q
> is unique (i.e the same at lower and higher values around M). I do not
> want the 37.5% above M and the 37.5% below. It makes a little
> difference with what is calculated with your routines.
> The idea would be to span the sample starting from the mean, and
> counting the points at lower and higher values around the mean in an
> iterative manner, until I have counted 75% of sample. This would give
> the value of Q at which the 75% is reached. I have a crude idea to do
> that with for loops but it will take forever...
>
> If you see what I mean, and if you have a piece of code, this could
> help a lot!
>
> Thanks again.
>
>
>> bing999 writes:
>>> I have sample of data (which distribution is unknown) of mean M. I
>>> would like to calculate the quartiles with IDL, i.e what is the value
>>> of Q for which 25% (or 75%) of the sample is comprised between [M-Q;M
>>> +Q] ?
>>> Do you know a routine which does that?
>>
>> cgBoxPlot.
>>
>> Cheers,
>>
>> David
>>
>> --
>> David Fanning, Ph.D.
>> Fanning Software Consulting, Inc.
>> Coyote's Guide to IDL Programming:http://www.idlcoyote.com/
>> Sepore ma de ni thui. ("Perhaps thou speakest truth.")
>

Easy enough (untested):

data = [......]
frac_to_enclose = 0.75
meanval = mean(data)
absdiff = abs(data-meanval)
quartile_index = floor(n_elements(absdiff) * frac_to_enclose)
q = absdiff[quartile_index]

But I share David's concern that this may not really be what you want...

-Jeremy.

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[Message index]

		Re: compute quartiles of a distribution By: Jeremy Bailin on Tue, 18 October 2011 21:32
		Re: compute quartiles of a distribution By: Jeremy Bailin on Tue, 18 October 2011 12:48
		Re: compute quartiles of a distribution By: Brian Wolven on Tue, 18 October 2011 10:38
		Re: compute quartiles of a distribution By: Thibault Garel on Tue, 18 October 2011 09:36
		Re: compute quartiles of a distribution By: David Fanning on Tue, 18 October 2011 09:25
		Re: compute quartiles of a distribution By: Thibault Garel on Tue, 18 October 2011 09:12
		Re: compute quartiles of a distribution By: Thibault Garel on Tue, 18 October 2011 09:13
		Re: compute quartiles of a distribution By: David Fanning on Mon, 17 October 2011 18:26
		Re: compute quartiles of a distribution By: Vincent Sarago on Mon, 17 October 2011 06:05

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