Re: [R] QQ plot for normality testing
I have found this web page useful http://www.cms.murdoch.edu.au/areas/maths/statsnotes/samplestats/qqplot.html Your mileage may vary. Keith J Matevz Pavlic matevz.pav...@gi-zrmk.si wrote in message news:ad5ca6183570b54f92aa45ce2619f9b901208...@gi-zrmk.si... Hi all, I am trying to test wheater the distribution of my samples is normal with QQ plot. I have a values of water content in clays in around few hundred samples. Is the code : qqnorm(w) #w being water content qqline(w) sufficient? How do I know when I get the plots which distribution is normal and which is not? Thanks, m [[alternative HTML version deleted]] __ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
Re: [R] QQ plot for normality testing
I would use the vis.test function along with vt.qqnorm (both in TeachingDemos package). This will create several plots, one of which is your data, the rest are simulated normals with the same mean and standard deviation. If you can tell which plot stands out (and it is your real data) then that suggests that the data is not normal. If you cannot tell which plot is the real data then that suggests that your data is close enough to normal. -- Gregory (Greg) L. Snow Ph.D. Statistical Data Center Intermountain Healthcare greg.s...@imail.org 801.408.8111 -Original Message- From: r-help-boun...@r-project.org [mailto:r-help-bounces@r- project.org] On Behalf Of Matevž Pavlic Sent: Saturday, April 30, 2011 11:28 AM To: r-help@r-project.org Subject: [R] QQ plot for normality testing Hi all, I am trying to test wheater the distribution of my samples is normal with QQ plot. I have a values of water content in clays in around few hundred samples. Is the code : qqnorm(w) #w being water content qqline(w) sufficient? How do I know when I get the plots which distribution is normal and which is not? Thanks, m [[alternative HTML version deleted]] __ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting- guide.html and provide commented, minimal, self-contained, reproducible code. __ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
Re: [R] QQ plot for normality testing
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.ps...@gmail.com] Sent: Saturday, April 30, 2011 8:04 PM To: Matevž Pavlič Cc: r-help@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.pav...@gi-zrmk.si wrote: Hi all, I am trying to test wheater the distribution of my samples is normal with QQ plot. I have a values of water content in clays in around few hundred samples. Is the code : qqnorm(w) #w being water content qqline(w) sufficient? How do I know when I get the plots which distribution is normal and which is not? Thanks, m [[alternative HTML version deleted]] __ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. -- Joshua Wiley Ph.D. Student, Health Psychology University of California, Los Angeles http://www.joshuawiley.com/ __ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
[R] QQ plot for normality testing
Hi all, I am trying to test wheater the distribution of my samples is normal with QQ plot. I have a values of water content in clays in around few hundred samples. Is the code : qqnorm(w) #w being water content qqline(w) sufficient? How do I know when I get the plots which distribution is normal and which is not? Thanks, m [[alternative HTML version deleted]] __ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
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.pav...@gi-zrmk.si wrote: Hi all, I am trying to test wheater the distribution of my samples is normal with QQ plot. I have a values of water content in clays in around few hundred samples. Is the code : qqnorm(w) #w being water content qqline(w) sufficient? How do I know when I get the plots which distribution is normal and which is not? Thanks, m [[alternative HTML version deleted]] __ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. -- Joshua Wiley Ph.D. Student, Health Psychology University of California, Los Angeles http://www.joshuawiley.com/ __ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
