Plot: Ellipse

The options described here are for customizing the ellipses plotted in a graph including the type of ellipse and the data used in calculating the ellipse. These options are available in the Ellipse dialog box, accessible by right-clicking on a point in a graph and selecting Ellipse from the shortcut menu. You can also access these options via the Graph Options dialog box by selecting the Ellipse tab (located under Plot).

Plot/Ellipse. Although all of the options are the same for each ellipse, you can make changes to each ellipse independently of the other. Select here the plot and ellipse to which you want to make changes. If you have several plots in one graph, you can assign names to each plot using the Name option in the General plot dialog box or on the General tab of the Graph Options dialog box. The two fields work together. For example, to edit the third ellipse in plot 2, set Plot to 2 and Ellipse to 3.

Add new ellipse/Delete. Add a new ellipse by clicking the Add button. When you do this, a new ellipse will be automatically added to the Ellipse list box, and that ellipse can then be defined using the options below. To delete an ellipse, select the appropriate plot and ellipse numbers in the Plot and Ellipse fields, then click Delete. Note that to delete plots, you must use the General plot dialog box or tab.

Type

Choose from two types of ellipses: Normal and Range.

Normal

This option will produce an ellipse based on the assumption that the two variables follow the bivariate normal distribution. The orientation of the ellipse is determined by the sign of the linear correlation between the two variables. The ellipse shows the prediction interval for a single new observation, given the parameter estimates for the bivariate distribution computed from the data, and the given n. Note that if the number of observations in the scatterplot is small, then the prediction interval may be very large, exceeding the area shown in the graph for the default scaling of the axes. Thus, in some cases (with small n) you may not see the prediction interval ellipse on the default graph (change the scaling to show larger intervals for the two variables in the plot). For additional information see, for example, Tracy, Young, and Mason (1992), or Montgomery 1996); see also the description of the prediction interval ellipse.

Range

This option will produce a fixed size ellipse such that the length of its horizontal and vertical projection onto the x- and y-axis (respectively) is equal to the mean ± (Range * I)/2 where the mean and range refer to the X or Y variable, and I is the current value of the coefficient field.

Coefficient

Specify the coefficient that controls the ellipses described above.

Pattern

Select the Pattern check box to create a line for the ellipse. You can customize the appearance of the line in the Line Properties dialog box, which is available by clicking the Pattern button. The current specifications for the line will be shown in the view box on the Pattern button.

Data

The Data box contains options regarding the scale to which the ellipse will be fit and the range of values to use when fitting the ellipse.

Scale

Select the scale to use in fitting the ellipse.

Range

You can choose from three ranges. If you choose Full Range, then the ellipse will be calculated using the entire range of the data. If your graph has a margin around the data, the calculations for the fit will not include points which are not contained in the data. If you select Axis range, the ellipse will be calculated using the entire range of the axis, regardless of what points are shown in the data. If you select Defined range, the ellipse will be applied over the range specified in the Min and Max fields.

Style

Instead of specifying line patterns using the Pattern option, you can select the style you want to use for this ellipse from the Style drop-down list box. To view a menu of options related to styles (Save, Save As, Revert to unmodified, etc.), click the button to the right of this box. For more information on styles, see Graphics Styles Overview.