Quartiles and Inter-Quartile Range

Why am I getting a wrong result for quartiles?

here is my code:
cbiomass = c(910, 1058, 929, 1103, 1056, 1022, 1255, 1121, 1111, 1192,
1074, 1415)
summary(cbiomass)
IQR(cbiomass)
The result R gives me is:

For the summary
Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
910    1048    1088    1104    1139    1415 

For IQR
91.25
*********

The true Q1 is 1039 and not 1048. Q2 is 1156.5 and not 1139. And the
Inter-Quartile Range is 117.5 and not 91.25

How can I get these number right?
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It's looks like you think that type=2 are the 'true' quantiles, but the
default method in R is type=7

You might want to look at ?stats::quantile

hth
david freedman
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Interestingly, Hmisc::describe() and summary() seem to be using one Type, and
stats::fivenum() seems to be using another Type.
fivenum(cbiomass)
[1]  910.0 1039.0 1088.5 1156.5 1415.0
summary(cbiomass)
Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
    910    1048    1088    1104    1139    1415
describe(cbiomass)$counts
n  missing   unique     Mean      .05      .10      .25      .50     
.75 
    "12"      "0"     "12"   "1104" " 920.5" " 938.3" "1047.5" "1088.5"
"1138.8" 
     .90      .95 
"1248.7" "1327.0" 

cheers,
-Girish
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Thank you David. I got it now.

Maybe you can help again.

I am typing the following code to draw a QQ plot but the line does not look right?

*******************
qqnorm(cbiomass)

qqline(cbiomass)
***********************
the graph looks very different from the graph generated by SAS.
There are two possibilities:
1. you need to use the argument 'datax = TRUE'
2. SAS is wrong

You might also have a look at the qq.plot() function
in package 'car'.

  -Peter Ehlers
Thank you again.

Emiliano.

Date: Fri, 22 Jan 2010 21:32:26 -0800
From: ml-node+1213199-1966337867 at n4.nabble.com
To: eeramalho at hotmail.com
Subject: Re: Quartiles and Inter-Quartile Range

It's looks like you think that type=2 are the 'true' quantiles, but the default method in R is type=7

You might want to look at ?stats::quantile

hth

david freedman

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