Probability-Probability Plots

Graphical Analytic Techniques

Ribbon bar. Select the Graphs tab. In the More group, click 2D and from the menu, select Probability-Probability Plots to display the Probability-Probability Plots Startup Panel.

Classic menus. On the Graphs - 2D Graphs submenu, select Probability-Probability Plots to display the Probability-Probability Plots Startup Panel.

The Probability-Probability (or P-P) plot is useful for determining how well a specific theoretical distribution fits the observed data.

In the P-P plot, the observed cumulative distribution function (the proportion of nonmissing values <= x) is plotted against a theoretical cumulative distribution function in order to assess the fit of the theoretical distribution to the observed data. If all points in this plot fall onto a diagonal line (with intercept 0 and slope 1), then you can conclude that the theoretical cumulative distribution approximates the observed distribution well. If the data points do not all fall on the diagonal line, then you can use this plot to visually assess where the data do and do not follow the distribution (e.g., if the points form an S shape along the diagonal line, then the data may need to be transformed in order to bring them to the desired distribution pattern).

In order to create this plot, the theoretical distribution function must be completely specified. Therefore, the parameters for the distribution must either be defined by the user or computed from the data (see the specified distribution for more information on the respective parameters). Note that you can also use the Quantile-Quantile plot to obtain the parameter estimate(s) (for the best fitting distribution from a family of distributions) to be used here.

The Probability-Probability Plots Startup Panel contains six tabs: Quick, Advanced, Appearance, Categorized, Options 1, and Options 2. The following options are available regardless of which tab is selected.

OK
Click the OK button to produce the graph.
Cancel
Click the Cancel button to close the Startup Panel without creating a graph.
Options
See Options Menu for descriptions of the commands on this menu.
By Group
Click the By Group button to display the By Group specification dialog box.
Sel Cond
Click the Sel Cond button to display the Analysis/Graph Case Selection Conditions dialog box, which is used to create conditions for which cases will be included (or excluded) in the current analysis. More information is available in the case selection conditions overview and syntax summary.

Case Weights. Click the Case Weights button to display the Analysis/Graph Case Weights dialog box, which is used to adjust the contribution of individual cases to the outcome of the current analysis by "weighting" those cases in proportion to the values of a selected variable.

Graphs Gallery
Click the Graphs Gallery button to display the Statistica Graphs Gallery dialog box, which is used to easily switch among different types of graphs.
Updating
Click the Updating button to display the following menu commands:
Auto
Select Auto to disconnect the graph from the graph creation dialog box and connect the graph directly to the spreadsheet. Any changes in the spreadsheet data (including selection condition changes) will be immediately reflected in the graph.
Manual
Select Manual to disconnect the graph from the graph creation dialog box and connect the graph directly to the spreadsheet. Any changes in the spreadsheet data will be reflected in the graph only after a manual update is initiated.
Locked
Select Locked to disconnect the graph from the graph creation dialog box and connect the graph directly to the spreadsheet. The graph is linked to the spreadsheet, but will not be updated when the spreadsheet data changes.  
Analysis
Select Analysis to connect the graph to the graph creation dialog box and disconnect the graph from the spreadsheet. Changes to analysis options (such as fit type) will be applied to all graphs created by the analysis.
Analysis/Updating
Select Analysis/Updating to connect the graph to the graph creation dialog box and disconnect the graph from the spreadsheet. The analysis (while open) will update the graph when the spreadsheet data is changed. Once the analysis is closed, the graph will no longer update when the spreadsheet data is changed.
None
Select None to disconnect the graph from both the spreadsheet and graph creation dialog box.

See also, Conceptual Overviews - Probability-Probability Plots.