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Quartiles and Inter-Quartile Range

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Quartiles and Inter-Quartile Range

eeramalho
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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Re: Quartiles and Inter-Quartile Range

David Freedman 2
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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Re: Quartiles and Inter-Quartile Range

Girish A.R.
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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Re: Quartiles and Inter-Quartile Range

David Freedman 2
and SAS give one a choice of 5 option, and i'm fairly sure that it used
a different default than does R (although one of the 5 corresponds to
the sas default)

see pctldef on
http://www.technion.ac.il/docs/sas/proc/z0146803.htm

my simple brain thinks of thinks of the problem as 'how does one
calculate the median of 4 values?'

david freedman

Girish A.R. [via R] wrote:

> 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
>
> View message @
> http://n4.nabble.com/Quartiles-and-Inter-Quartile-Range-tp1145817p1248728.html 
>
> To unsubscribe from Re: Quartiles and Inter-Quartile Range, click here
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RE: Quartiles and Inter-Quartile Range

eeramalho
In reply to this post by David Freedman 2
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.

Thank you again.

Emiliano.


Date: Fri, 22 Jan 2010 21:32:26 -0800
From: [hidden email]
To: [hidden email]
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


View message @ http://n4.nabble.com/Quartiles-and-Inter-Quartile-Range-tp1145817p1213199.html
To unsubscribe from Quartiles and Inter-Quartile Range, click here.



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Re: Quartiles and Inter-Quartile Range

David Freedman 2
please do some reading - I *think* the main difference is that the x and y axes are reversed, but I really don't know what SAS prints out

there are a many 'defaults' that are rather arbitrary - sometimes SAS uses 1, while R uses another

??qqnorm brings up a list of functions, including stats::qqnorm
?stats::qqnorm brings up the help page for the function

On Mon, Jan 25, 2010 at 12:41 PM, eeramalho [via R] <[hidden email]> wrote:
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.

Thank you again.

Emiliano.


Date: Fri, 22 Jan 2010 21:32:26 -0800
From: [hidden email]
To: [hidden email]
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


View message @ http://n4.nabble.com/Quartiles-and-Inter-Quartile-Range-tp1145817p1213199.html
To unsubscribe from Quartiles and Inter-Quartile Range, click here.



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--
Natalia and/or David

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Re: [R] Quartiles and Inter-Quartile Range

Peter Ehlers
In reply to this post by eeramalho

eeramalho wrote:

>
> 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: [hidden email]
> To: [hidden email]
> 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
>
>
>
>
>
> View message @ http://n4.nabble.com/Quartiles-and-Inter-Quartile-Range-tp1145817p1213199.html
>
>
> To unsubscribe from Quartiles and Inter-Quartile Range, click here.
>
>
>      
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