Contents - Index

Select/View Distribution Dialog Box



This dialog box is used to specify a distribution and its parameters.  It is used by the Generate Random Values dialog box to select a distribution to use when generating random numbers.  It is also used by the Skewness-Kurtosis Plot window to view different distributions to better understand their shapes.  The following options can be set:  

Distribution/Family Radio Group:  Select the distribution or family of distributions to use from the list.  The following are available: beta, exponential, negative exponential, largest extreme value, smallest extreme value, gamma, Johnson, loglogistic, lognormal, normal, Pearson and uniform.

Moments Group:  Used to enter up to four moments depending on the distribution.  All distributions require the first two moments (average and standard deviation).  Many also require the skewness.  A few require both the skewness and kurtosis.

Average Edit Box:  First moment required for all distributions.  Must be a real number.

Standard Deviation Edit Box:  Second moment required for all distributions.  Must be a real number greater than zero.

Skewness Edit Box:  Third moment.  Only displayed for certain distributions.  Can be any real number except for the largest extreme value (> -5.6051382), smallest extreme value (< 5.6051382) and loglogistic (> -4.284783 and < 4.284783).

Kurtosis/Excess Kurtosis Edit Box:  Fourth moment.  Only displayed for certain distributions.  Enter either the kurtosis or excess kurtosis depending on which is selected in the Analysis Options dialog box.  The kurtosis must be a real number greater than 1.0 and satisfying Kurtosis > Skewness * Skewness + 1.  The excess kurtosis must be a real number greater than -2.0 and satisfying Excess Kurtosis > Skewness * Skewness - 2.  Otherwise, the specified value falls into the impossible region.

X Edit Box:  Used to explore the distribution.  Enter a value for X and the percent of values less than X will be displayed (distribution function).  For the distribution shown above, 63.2% of values fall below 0.  

P Edit Box:  Used to explore the distribution.  Enter a percentage for P and the value of X that has P% of values below it will be displayed (inverse of distribution function).  For the example above 50% of the values fall below -0.31.

A picture of the selected distribution is displayed if the moments are valid.  Otherwise, an error message is displayed.  The parameters of the distribution are also given on a second tab.  Right clicking the mouse while the cursor is over one of these tabs displays a popup menu for printing the graphic or copying it to the clipboard.

When done, click the OK button.  If any errors are found, an error message will be displayed and the errors must be corrected before the dialog box can be closed.  To exit without changing the distribution, click the Cancel button or press the Esc key.

This dialog box is displayed by clicking the Select Distribution button on the Generate Random Values dialog box and by clicking the View Distributions button on the Skewness-Kurtosis Plot window.