| Re: Box-Whisker plots in IDL [message #55390 is a reply to message #55389] |
Mon, 20 August 2007 16:47   |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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jschwab@gmail.com writes:
> Pardon me if I'm mistaken, but I think these "quartiles with
> histogram" examples, including the one that's in JD's histogram
> tutorial are fundamentally incorrect.
>
> You are assuming "Equal bin widths" ==> "Equal #'s in each bin" !
>
> When HISTOGRAM splits a data list into N bins, it does so such that
> the *width* of the bins are equal. In no way does it somehow create a
> situation in which the *number of points* in each bin is equal (which
> is what would be required to find quartiles in such a manner).
>
> The given examples have only "worked" because you're either dealing
> with uniform distributions (in which case equal bin widths do imply
> equal numbers in each bin) or because the example data happens to be
> roughly uniform.
>
> If you want to convince yourself, try one of those codes with
> data = randomu(seed, 1000) * 100.
> and then with
> data2 = data * data
> The quartiles in the 2nd case should simply be the squares of the
> quartiles from the first.
Humm. Maybe you are right. (Isn't it odd that math types
never hit the SEND button until someone else has made
a fool of themselves?)
OK, how about this:
data=randomu(sd,100)*100
minVal = min(data)
maxVal = max(data)
medianVal = median(data,/even)
; Find the quartiles.
qtr_25th = Median(data[Where(data LE medianVal, countlowerhalf)])
qtr_75th = Median(data[Where(data GT medianVal, countupperhalf)])
void = Where(data LT qtr_25th, countlowerquarter)
void = Where(data GE qtr_75th, countupperquarter)
Print, minVal, maxVal, medianVal, qtr_25th, qtr_75th
Print, countlowerquarter, countlowerhalf-countlowerquarter, $
countupperhalf-countupperquarter, countupperquarter
END
Which gives me:
1.74060 99.8840 53.5631 31.7422 73.8378
25 25 25 25
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Sepore ma de ni thui. ("Perhaps thou speakest truth.")
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