Next: Correlation and Regression Analysis, Previous: Basic Statistical Functions, Up: Statistics [Contents][Index]

Octave can create Quantile Plots (QQ-Plots), and Probability Plots (PP-Plots). These are simple graphical tests for determining if a data set comes from a certain distribution.

Note that Octave can also show histograms of data
using the `hist`

function as described in
Two-Dimensional Plots.

- Function File:
*[*`q`,`s`] =**qqplot***(*`x`) - Function File:
*[*`q`,`s`] =**qqplot***(*`x`,`y`) - Function File:
*[*`q`,`s`] =**qqplot***(*`x`,`dist`) - Function File:
*[*`q`,`s`] =**qqplot***(*`x`,`y`,`params`) - Function File:
**qqplot***(…)* Perform a QQ-plot (quantile plot).

If F is the CDF of the distribution

`dist`with parameters`params`and G its inverse, and`x`a sample vector of length`n`, the QQ-plot graphs ordinate`s`(`i`) =`i`-th largest element of x versus abscissa`q`(`i`f) = G((`i`- 0.5)/`n`).If the sample comes from F, except for a transformation of location and scale, the pairs will approximately follow a straight line.

If the second argument is a vector

`y`the empirical CDF of`y`is used as`dist`.The default for

`dist`is the standard normal distribution. The optional argument`params`contains a list of parameters of`dist`. For example, for a quantile plot of the uniform distribution on [2,4] and`x`, useqqplot (x, "unif", 2, 4)

`dist`can be any string for which a function`distinv`or`dist_inv`exists that calculates the inverse CDF of distribution`dist`.If no output arguments are given, the data are plotted directly.

- Function File:
*[*`p`,`y`] =**ppplot***(*`x`,`dist`,`params`) Perform a PP-plot (probability plot).

If F is the CDF of the distribution

`dist`with parameters`params`and`x`a sample vector of length`n`, the PP-plot graphs ordinate`y`(`i`) = F (`i`-th largest element of`x`) versus abscissa`p`(`i`) = (`i`- 0.5)/`n`. If the sample comes from F, the pairs will approximately follow a straight line.The default for

`dist`is the standard normal distribution.The optional argument

`params`contains a list of parameters of`dist`.For example, for a probability plot of the uniform distribution on [2,4] and

`x`, useppplot (x, "uniform", 2, 4)

`dist`can be any string for which a function`dist_cdf`that calculates the CDF of distribution`dist`exists.If no output is requested then the data are plotted immediately.