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How to use prctile in grpstats?

Asked by Lei on 15 May 2013

Hello, I am using the grpstats function to get some statistic based on groups; however, I have trouble to use prctile for the whichstats. As the prctile requires a both input X and p. The p I want is [ 5 95]. But for the input x, I don't know how to handle with this as I have predefined X in the grpstats. Here is my grpstats input example:

[A,B,C,D,E,F] = grpstats(sample(:,2),sample(:,7),{'gname','numel','mean','var','skewness',@iqr});

Sample(:,2) is my input data, and sample(:,7) is the group. I am wondering how can I use the prctile in the grpstats properly. Thanks.

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Lei

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1 Answer

Answer by Tom Lane on 16 May 2013
Accepted answer

The trick is to use an anonymous function that has the desired percentiles built in. For example, the following two calls to grpstats yield the same results:

x = randn(1000,2);
g = randi(5,1000,1);
grpstats(x,g, @median)
grpstats(x,g, @(x) prctile(x,50))

The following yields a 3-D array where the middle page, a(:,:,2), has the median and the other pages have the 5% and 90% points:

a = grpstats(x,g, @(x) prctile(x,[5 50 90]))

3 Comments

Lei on 16 May 2013

Hello Tom, Thanks for your help! your code works well. I just do not quite understand the @(x). Apparently,this is the part I got wrong. Would you please explain a bit more? thx again. Regards.

Tom Lane on 16 May 2013

That's called an anonymous function. It's a way to define a function on the fly, at the command line, without entering code into a file. So "@(x)prctile(x,50)" is a function of x that is computed by invoking the prctile function with x as the first input and 50 as the second.

Lei on 16 May 2013

I see. Thx!

Tom Lane

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