Re: [netcdfgroup] Calculating medians, quartiles etc of a variable in a NetCDF file.

I think the reason why functions for calculating quartiles (or more generally, 
percentiles) are not in many software packages is because they can be 
computationally expensive to compute. 

The only way I know how to generate percentiles is to first sort the data into 
numeric order, and then select the appropriate value from the list. Sorting is 
expensive, especially when there are a lot of data. Is there a better/faster 
way of computing percentiles?

Tim Hume
Centre for Australian Weather and Climate Research
Melbourne
Australia 

-----Original Message-----
From: Gerry Creager [mailto:gerry.creager@xxxxxxxx] 
Sent: Monday, 12 May 2008 12:41
To: Timothy Hume
Cc: Dennis Shea
Subject: Re: [netcdfgroup] Calculating medians, quartiles etc of a variable in 
a NetCDF file.

I'd consider looking at the NCAR command language and seeing if quartile 
calculation is native therein.  I'd be surprised if it's not, or easily 
computable, but I did not find it initially.

Check http://www.ncl.ucar.edu/ and consider joining the mailing list 
long enough to find out.



Timothy Hume wrote:
> Hi,
> 
> I have an application where I would like to calculate quartiles of a variable 
> in a NetCDF file (I am computing the trimean: TM = 0.25*(Q1 + 2Q2 + Q3) )
> 
> The NetCDF operators are excellent for computing arithmetic means, but don't 
> include the capability to compute quartiles. Before I start writing my own 
> code to compute quartiles, does anyone know of some software which already 
> does this? Basically, I am looking for the equivalent of ncwa with a quartile 
> operation.
> 
> Thanks,
> 
> Tim Hume
> Centre for Australian Weather and Climate Research
> Melbourne
> Australia 
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-- 
Gerry Creager -- gerry.creager@xxxxxxxx
Texas Mesonet -- AATLT, Texas A&M University    
Cell: 979.229.5301 Office: 979.458.4020 FAX: 979.862.3983
Office: 1700 Research Parkway Ste 160, TAMU, College Station, TX 77843


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