[R] Simple qqplot question

[Joris Meys]

Sorry, missed the two variable thing. Go with the lm solution then,
and you can tweak the plot yourself (the confidence intervals are
easily obtained via predict(lm.object, interval="prediction") ). The
function qq.plot uses robust regression, but in your case normal
regression will do.

Regarding the shapes : this just indicates both tails are shorter than
expected, so you have a kurtosis greater than 3 (or positive,
depending whether you do the correction or not)

Cheers
Joris

On Fri, Jun 25, 2010 at 4:10 AM, Ralf B <ralf.bierig at gmail.com> wrote:
> 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?
>
> |
> |                               *  *   *  *
> |                           *
> |                        *
> |                    *
> |               *
> |            *
> |         *
> | * * *
> |---------------------------------------------
>
> Thanks, Ralf
>



-- 
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