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The figure shows the Q-Q plot of a theoretical and empirical standardized Normal distribution generated through the $qqnorm()$ function of R statistical tool.

enter image description here

How can I describe the right tail (top right) that does not follow the red reference line? What does it mean when there is a "trend" that running away from the line?

Thank you

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    @Michael Hardy, yes my data are not coming from rnorm :) Sorry I forgot to specify it!2011-08-27

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It means that in the right tail, your data do not fit normal well, specifically, there are far less numbers there would be in a normal sample. If the black curved up, there would be more than in a typical normal sample.

You can think of the black curve as a graph of a function that , if applied to your data, would make them like a normal sample.

In the following image, random sample is generated by applying Ilmari Karonen's function to normal sample.

Screen shot of Mathematica session

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    @$M$aurizio You got it right, I am merely rephrasing your statement to make it slightly more precise. The count of points at the right extreme in your data is lower, than expected in a sample drawn from normal distribution.2011-08-26
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Looks like your data has a cutoff at $4$. You could probably fit the samples you plotted fairly well with a curve such as

$y = \frac 1 2 \left( x + 4 - \sqrt{c+(x-4)^2} \right),$

where $c > 0$ is a free parameter that describes the sharpness of the cutoff. Just by eyeballing the graph, I'd guess that $c \approx 0.1$ for your data.

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    i don t $k$now why, i'm analyzing the distribution of a voip traffic generator that should follows a normal distribution but first i have to understand how can i "read" the qq plots :)2011-08-26