[R] QQ plot for normality testing

[Matevž Pavlič]

Thanks for the answer and for the link. I was lookin for a search trough the forum posts....

So the slope of the line is not important as long as the data is approx. on the line?
Thanks, m

-----Original Message-----
From: Joshua Wiley [mailto:jwiley.psych at gmail.com] 
Sent: Saturday, April 30, 2011 8:04 PM
To: Matevž Pavlič
Cc: r-help at r-project.org
Subject: Re: [R] QQ plot for normality testing

Hi,

qqnorm basically plots your actual sample values against what the values would be (approximately) if they were from a normal distribution.  qqline() adds a line through the 1st and 3rd quartiles.
 So roughly speaking, if your QQ plot forms a straight line (particularly the one drawn by qqline), then your sample values match a normal distribution.  With real data, it is typically not a "yes/no"
decision, rather "is my data normal enough?"

Questions like this have been asked many times on this list, so searching the mailing list archives will lead you to many more discussions and suggestions.  Here is one way to search:
http://tolstoy.newcastle.edu.au/R/

Cheers,

Josh


On Sat, Apr 30, 2011 at 10:27 AM, Matevž Pavlič <matevz.pavlic at gi-zrmk.si> wrote:
> Hi all,
>
>
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> I am trying to test wheater the distribution of my samples is normal with QQ plot.
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> I have a values of water content in clays in around few hundred samples. Is the code :
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> qqnorm(w)      #w being water content
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> qqline(w)
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> sufficient?
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> How do I know when I get the plots which distribution is normal and which is not?
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> Thanks, m
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>
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--
Joshua Wiley
Ph.D. Student, Health Psychology
University of California, Los Angeles
http://www.joshuawiley.com/