[R] strange results in summary and IQR functions

[Erik Iverson]

It's all simply a matter of definitions, and there are many who disagree. See ?quantile , specifically the "type" argument.  Since IQR does not appear to have a type argument, you could easily write your own versions of these that do what SAS does (assuming that is your goal).  

With x defined as you have it, look at the results of this function call, which shows the different values for quantile that you get by using different "type" arguments.

> sapply(1:9, function(y) quantile(x, type = y))

     [,1] [,2] [,3] [,4] [,5]  [,6]  [,7]     [,8]    [,9]
0%      2    2    2  2.0    2  2.00  2.00  2.00000  2.0000
25%    11   11    4  7.5   11  9.25 11.25 10.41667 10.5625
50%    13   14   13 13.0   14 14.00 14.00 14.00000 14.0000
75%    31   31   31 31.0   31 32.50 31.00 31.50000 31.3750
100%   47   47   47 47.0   47 47.00 47.00 47.00000 47.0000


Best,
Erik Iverson 


-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of Chunhao Tu
Sent: Tuesday, September 08, 2009 10:09 AM
To: r-help at r-project.org
Subject: [R] strange results in summary and IQR functions


Dear R users,
Something is strange in summary and IQR. Suppose, I have a data set and I
would like to find the Q1, Q2, Q3 and IQR.  
 
x<-c(2,4,11,12,13,15,31,31,37,47)
> summary(x)
   Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
   2.00   11.25   14.00   20.30   31.00   47.00 
> IQR(x)
[1] 19.75
However, I test the same data set in SAS "proc univariate", and SAS shows
that Q1=11, Q2=14 and Q3=31. I think most of us agree that Q1 is 11 not
11.25. 

Could someone please explain to me why R shows Q1=11.25 not 11?

Many Thanks
Tu


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