Re: [R] Simple qqplot question

From: Ralf B <ralf.bierig_at_gmail.com>
Date: Thu, 24 Jun 2010 23:22:26 -0400

Short rep: I have two distributions, data and data2; each build from about 3 million data points; they appear similar when looking at densities and histograms. I plotted qqplots for further eye-balling:

qqplot(data, data2, xlab = "1", ylab = "2")

and get an almost perfect diagonal line which means they are in fact very alike. Now I tried to check normality using qqnorm -- and I think I am doing something wrong here:

qqnorm(data, main = "Q-Q normality plot for 1") qqnorm(data2, main = "Q-Q normality plot for 2")

I am getting perfect S-shaped curves (??) for both distributions. Am I something missing here?

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Thanks, Ralf

On Thu, Jun 24, 2010 at 8:23 PM, Ralf B <ralf.bierig_at_gmail.com> wrote:

> Unfortunately not. I want a qqplot from two variables.
>
> Ralf
>
>
> On Thu, Jun 24, 2010 at 7:23 PM, Joris Meys <jorismeys_at_gmail.com> wrote:
>> Also take a look at qq.plot in the package "car". Gives you exactly
>> what you want.
>> Cheers
>> Joris
>>
>> On Fri, Jun 25, 2010 at 12:55 AM, Ralf B <ralf.bierig_at_gmail.com> wrote:
>>> More details...
>>>
>>> I have two distributions which are very similar. I have plotted
>>> density plots already from the two distributions. In addition,
>>> I created a qqplot that show an almost straight line. What I want is a
>>> line that represents the ideal case in which the two
>>> distributions match perfectly. I would use this line to see how much
>>> the errors divert at different stages of the plot.
>>>
>>> Ralf
>>>
>>>
>>>
>>> On Thu, Jun 24, 2010 at 5:56 PM, stephen sefick <ssefick_at_gmail.com> wrote:
>>>> You are going to have to define the question a little better.  Also,
>>>> please provide a reproducible example.
>>>>
>>>> On Thu, Jun 24, 2010 at 4:44 PM, Ralf B <ralf.bierig_at_gmail.com> wrote:
>>>>> I am a beginner in R, so please don't step on me if this is too
>>>>> simple. I have two data sets datax and datay for which I created a
>>>>> qqplot
>>>>>
>>>>> qqplot(datax,datay)
>>>>>
>>>>> but now I want a line that indicates the perfect match so that I can
>>>>> see how much the plot diverts from the ideal. This ideal however is
>>>>> not normal, so I think qqnorm and qqline cannot be applied.
>>>>>
>>>>> Perhaps you can help?
>>>>>
>>>>> Ralf
>>>>>
>>>>> ______________________________________________
>>>>> R-help_at_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.
>>>>>
>>>>
>>>>
>>>>
>>>> --
>>>> Stephen Sefick
>>>> ____________________________________
>>>> | Auburn University                                   |
>>>> | Department of Biological Sciences           |
>>>> | 331 Funchess Hall                                  |
>>>> | Auburn, Alabama                                   |
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>>>> |___________________________________|
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>>>> | http://www.auburn.edu/~sas0025             |
>>>> |___________________________________|
>>>>
>>>> Let's not spend our time and resources thinking about things that are
>>>> so little or so large that all they really do for us is puff us up and
>>>> make us feel like gods.  We are mammals, and have not exhausted the
>>>> annoying little problems of being mammals.
>>>>
>>>>                                                                -K. Mullis
>>>>
>>>
>>> ______________________________________________
>>> R-help_at_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.
>>>
>>
>>
>>
>> --
>> Joris Meys
>> Statistical consultant
>>
>> Ghent University
>> Faculty of Bioscience Engineering
>> Department of Applied mathematics, biometrics and process control
>>
>> tel : +32 9 264 59 87
>> Joris.Meys_at_Ugent.be
>> -------------------------------
>> Disclaimer : http://helpdesk.ugent.be/e-maildisclaimer.php
>>
>

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