How to use prctile in grpstats?

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Lei
Lei 2013-5-15
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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Tom Lane
Tom Lane 2013-5-16
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]))
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Tom Lane
Tom Lane 2013-5-16
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.

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