R help - Jan 2010 - 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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