Problem in calculating 30th percentile

Hello everyone,

I am trying to calculate the 30th percentile. I tried using prctile function but didn't get the result. I've calculated 50th percentile (median) as below:

Temp = 437x1 double:
Area = 437x1 double;

[G,grpTemp] = findgroups(round(Temp));  
medianAr = splitapply(@median,Area,G);

Any suggestions how to calculate 30th percentile?

Answer 1

[Rik]

Write a function that calculates the 30th percentile for a vector. Then provide the handle to that function to splitapply.

The easiest way to calculate the 30th percentile is probably to sort your vector and index it. Note that you may need to average 2 values if the index is not an integer.

[Star Strider]

The prctile function should work. I wonder what the problem is with it?

I would use sort and interp1 instead of the index, unless the index (and not the exact value) is required. That should provide a reasonable approximation.

[Zhou Ci]

@Star Strider I tried doing like this, but it didn't work

Y = splitapply(@prctile,[30], Area,G);

I didn't understand the purpose of using sort and interp1 here. Kindly explain.

[Rik]

splitapply(@(x) prctile(x,30),Area,G)

And the other remarks were about my suggestion to write your own function.

[Zhou Ci]

Thanks!