# How is a q-q plot different from a quantile plot?

Hi. What is the difference between a quantile plot and a q-q plot?
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Commented:
A q-q plot is a quantile - quantile plot  which plots the quantiles of the density function in question against a known density function.  This is useful in checking distributions for fit.  Sometimes a q-q plot is refered to as a quantile plot - but this is not fully correct

A quantile plot is a plot which plots the quantiles of the density function in question against the explanatory density.

Background for q-q plot  http://en.wikipedia.org/wiki/q-q_plot

A fairly easy reference for a pure quantile plot can be found here:
http://home.jesus.ox.ac.uk/~clifford/a5/chap2/node4.html

Background for quantile plot.

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Commented:
A Q-Q plot is a plot "of quantiles"

>>>> A Q-Q plot is a plot of the quantiles <<<<
of two distributions against each other, or a plot based on estimates of the quantiles. The pattern of points in the plot is used to compare the two distributions.

The main step in constructing a Q-Q plot is calculating or estimating the quantiles to be plotted. If one or both of the axes in a Q-Q plot is based on a theoretical distribution with a continuous cumulative distribution function (CDF), all quantiles are uniquely defined and can be obtained by inverting the CDF. If a theoretical probability distribution with a discontinuous CDF is one of the two distributions being compared, some of the quantiles may not be defined, so an interpolated quantile may be plotted. If the Q-Q plot is based on data, there are multiple quantile estimators in use. Rules for forming Q-Q plots when quantiles must be estimated or interpolated are called plotting positions.
http://en.wikipedia.org:80/wiki/Q-Q_plot
Commented:
All of which is true but did not answer the posted question which was about the difference between two different plots.

One (the Q-Q) plots the data distribution against a known distribution. The other (quantile) plots the CDF against the independent variable... that is it just shows the empirical distribution without comparing it to ANY distribution. BTW - thanks for repeating my link for the q-q plot in your answer, as if graziazi couldn't see it the first time. I won't have to repeat it here a third time.
Commented:
I'm interpeting the info differently, and maybe wrongly.
The poster asked the difference between a quantile plot and a Q-Q plot.
My interpetation of the info is that they are not both plots.
A Q-Q plot is a plot "of quantiles". I'm not reading anything that identifies a quantile plot. A Q-Q plot is a "plot of quantiles" thus the way I'm reading it there is no such thing as a quantile plot. There are "plots of quantiles" and that plot is called a Q-Q plot.
Commented:
Yes nick, there is such a thing as a quantile plot. (Take a look at my original post) Well, there was one link above you could look at. This following link will show a nice quantile graph.

http://www.stata.com/support/faqs/graphics/gph/graphdocs/dist2.html

A quantile graph or plot is like a cumulative histogram.  The CDF is the same as the quantile graph for a function and is the term most often used by people working with statistics. A Q-Q or quantile-quantile graph is NOT a quantile graph, but people often get lazy and call it one since the data shown by a quantile graph is usually shown by graphing the CDF. Who wants to say quantile-quantile when a simple 'quantile' will do for them?

However, for a new student trying to learn a new topic, the sloppy use of the language can be very confusing. What one person "knows" to be a short version of a longer phrase, to the new student the short version means, potentially, something else.   Although not in great use, a quantile plot does exist and should not be mixed with a quantile-quantile plot  (which is better called just a q-q plot)

One correction to my original post. Where I said "against the explanatory density" in the second paragraph. It should have read "against the explanatory variable"
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