Median calculation gives dodgy result

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Median calculation gives dodgy result #468108 09/18/17 21:43 09/18/17 21:43
Joined: Aug 2017 Posts: 40 J johnnyp OP Newbie
johnnyp OP Newbie J Joined: Aug 2017 Posts: 40	Code: var a[4] = {1, 4, 3, 5}; printf("n%f", Median(a, 4)); printf("n%f", Percentile(a, 4, 50)); # output 4.500000 4.500000 Unless I am much mistaken the median value should be 3.5 Median() always gives the right answer when length is even, and never when length is odd. Last edited by johnnyp; 09/18/17 22:01.

Re: Median calculation gives dodgy result [Re: johnnyp] #468111 09/19/17 00:06 09/19/17 00:06
Joined: Sep 2017 Posts: 7 TX F flare9x Newbie
flare9x Newbie F Joined: Sep 2017 Posts: 7 TX	Interesting - may have a bug there. I tested in R: Code: x <- c(1,3,4,5) median(x) [1] 3.5 Can you try maybe the R bridge, maybe the R function can calculate it correctly? Last edited by flare9x; 09/19/17 00:08.

Re: Median calculation gives dodgy result [Re: flare9x] #468125 09/19/17 09:55 09/19/17 09:55
Joined: Jul 2000 Posts: 27,982 Frankfurt jcl Chief Engineer
jcl Chief Engineer Joined: Jul 2000 Posts: 27,982 Frankfurt	I would also expect that the median in that example is 3.5 and not 4.5. The Median function calls the Percentile function and the Percentile function is from some statistics library. Maybe that library does not consider even lengths. I'll forward that to the developers to check.

Re: Median calculation gives dodgy result [Re: jcl] #468129
09/19/17 12:53 09/19/17 12:53

Joined: Aug 2017
Posts: 40

johnnyp

OP
Newbie

johnnyp

OP
Newbie

Joined: Aug 2017
Posts: 40

I wrote my own version of Percentile.

Code:

var percentile(vars data, int length, var percent)
{
	int* ids = sortIdx(data, length);
	var pos = (length-1)*percent/100;
	var below = data[ids[min(length-1, (int)pos)]];
	if(pos == round(pos))
		return below;
	var above = data[ids[min(length-1, (int)pos+1)]];
	return (below+above)/2;
}

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