The example model used in this section, Reliability.xls, shows the reliability of a design component when it is manufactured from different materials and is subjected to varying stresses.
Each forecast chart displays the design component’s reliability as a ratio of two distributions, strength and stress. A reliable component has values greater than 1, because the component’s strength exceeds the stress subjected to it. An unreliable component has values less than 1, because its strength is less than the stress. For this model, the overlay chart can be used to compare component reliabilities.
To create an overlay chart:
Run a simulation in Crystal Ball.
To produce a meaningful overlay chart, the simulation should have more than one forecast. Reliability.xls is used in these instructions.
Choose Analyze, then Overlay Charts.
(If you are using Crystal Ball in Microsoft Excel 2007 or later, choose Analyze, then View Charts, then Overlay Charts.)
The Overlay Charts dialog opens. If you have not yet created any overlay charts for open workbooks or restored results files, the dialog is blank.
To create a new overlay chart, click New.
The Choose Forecasts dialog opens.
By default, this dialog opens in a hierarchical Tree view. If you prefer, click the List button to change the view from a tree to a list.
Check the boxes in front of the forecasts to include in the overlay chart.
Click OK to create a new overlay chart with the selected forecasts as shown in Figure 37, Overlay Chart for Selected Forecasts.
The overlay chart is displayed with the frequency distributions for the selected forecasts superimposed over each other. By default, they are displayed as columns. You can change them to lines or areas in two or three dimensions and can change the number of data points or intervals (bins) with the Chart Preferences settings (Setting Chart Preferences). A default name is assigned; you can also change it with the Chart Preferences settings.
Follow the steps in Customizing Overlay Charts and Setting Chart Preferences to change a variety of chart features and highlight those of greatest interest.
You can also use "shortcut keys," or keyboard equivalents for commands to quickly change the chart preferences. For a list of these, see Table 5, Shortcut keys for Chart Preferences .
In this example, press Ctrl+d until the reverse cumulative chart view is displayed. Then, press Ctrl+t until the outline chart type is displayed.
As shown in Figure 38, Overlay Chart with Three Distributions, the chart view changes to show all three distributions completely. This chart in outline view most clearly suggests that Material 3 has superior reliability and is dominant since a greater proportion of its distribution is to the right of 1.00 and its values for all probability levels are higher than the others.
Now, experiment with chart shortcut keys.
Press Ctrl+d again until the frequency distribution is displayed. Press Ctrl+t to display the column chart. Try pressing Ctrl+b to change the number of frequency bins (columns, in this view). Then, press Ctrl+w to make the chart three-dimensional.
If you want, you can drag either side of the chart to give it a taller, narrower look, as shown in Figure 39, Overlay Chart, 3D View, or a stretched look as in Figure 40, A Rotated and Stretched Overlay Chart.
In 3D view, the Enable Rotation checkbox is displayed at the top of the chart, accessible with the Tab key. When it is checked, you can click inside the chart and drag to rotate it. This can enhance the data display for both analysis and presentation. Figure 40, A Rotated and Stretched Overlay Chart shows a rotated overlay chart, stretched to emphasize x-axis differences.
