# Probability Distribution Calculator

Select Probability calculator on the Basic Statistics and Tables - Quick tab to display the Probability Distribution Calculator. This facility enables you to compute, for various theoretical distributions and parameters (e.g., F, df1, df 2), either a) the upper or lower tail areas, given a user-specified variate value (two-sided probability values can be computed for symmetrical distributions; e.g., the normal distribution), or b) the critical values, given a user specified probability value. See also the Probability Distribution Calculator Overview.

Distributions and Parameters. A variety of distributions are available in the Probability Distribution Calculator. For more details on the parameters for a specific distribution, click the distribution below.

Fixed Scaling. This option affects the scaling of the (continuously updated) graph depicted in the icons at the bottom of the dialog box, and the graphs that are produced if you select the Create graph check box. When the Fixed Scaling check box is selected, Statistica will use a fixed scale for the graphs, and as you change the parameters of the respective distribution, you can observe the changes in the graph relative to the same fixed scales. Note that for extreme parameter settings, this may result in the graphs "running-off the screen." To avoid this, clear the Fixed Scaling check box, which causes the program to recompute the optimum scaling for the graphs for each parameter setting, resulting in optimum resolution. However, for some distributions, this will give the appearance that the shapes of the functions are not changing in response to different parameter settings; this happens, for example, when you change the standard deviation parameter for the normal (z) distribution when the Fixed Scaling check box is cleared.

Inverse. Select this check box to compute the inverse of the integral; that is, you can enter a desired significance level, and Statistica computes the critical value of the respective statistic. Note that if you change the value of p in the respective edit field, this option will automatically be selected.

Two-tailed. Select this check box to specify a two-tailed test. If this check box is cleared, a one-tailed test is performed. This option is only available for symmetric distributions.

(1-Cumulative p). Select this check box to compute the critical value for the specified distribution based on a (1-cumulative p) value or the (1-cumulative p) value based on a specified critical value.

Send to Report. Select this check box to send the p-value and critical value for the specified distribution and parameters to a report. These values are sent to a report when you click the Compute button, not when you change the parameters with the microscrolls. Therefore, you can edit the parameters using the microscrolls and review the results in the boxes in this dialog box; then, when you are ready, click the Compute button to send the resulting values to a report. Note that this feature will not work unless you have selected the Also send to Report Window check box on the Options dialog box - Output Manager tab. If that check box is not selected, you will be prompted to select it before the resulting values can be sent to a report.

Create Graph. Select this check box if you want the Probability Density Function and Cumulative Distribution Function graphs for the specified distribution and parameters to be placed in a compound graph. The graph is created when you click the Compute button, not when you change the parameters with the microscrolls. Therefore, you can edit the parameters using the microscrolls and review the results in the graph icons in this dialog box; then, when you are ready, click the Compute button to create the desired graph. The compound graph is displayed in an individual window regardless of the setting on the Options dialog box - Output Manager tab or in the Analysis/Graph Output Manager dialog box.

Compute. Click this button to enter the value currently edited in an edit box (if microscrolls are not used) and compute the p-value or critical value for the specified distribution based on the distribution's parameters. When the Send to Report check box is selected, clicking this button will also send the resulting critical value and p-value to a report. When the Create Graph check box is selected, clicking this button will create a compound graph containing the Probability Density Function and Cumulative Distribution Function for the specified distribution and parameters.

Exit. Click this button to exit the dialog box.