Bag Plots - Advanced Tab

Graphical Analytic Techniques

The Advanced tab of the Bag plot dialog contains a variety of options for creating bag plots.

Variables. Click the Variables button to display the standard variable selection dialog, in which you select the variable(s) to be plotted. When you select more than one variable a sequence of graphs is produced (one for each of the variables). The selection that you make will then be displayed in the area of the dialog below the Variables button. If you select marked subsets, then the method of categorization selected will apply to all bag plots.

Graph type. Select the type of bag plot to be plotted from the Graph type list. Click the type of the plot link listed below for a brief description of that type of graph.

Ellipse. Use the options under Ellipse to superimpose an ellipse on the data in the bag plot. You can display one of two types of ellipses (Normal or Range, see below), or leave it off (select Off).

Normal. Select this option button to 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 longer axis of the ellipse is superimposed on the regression line). 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. Select this option button to produce a fixed size ellipse such that the length of its horizontal and vertical projection onto the x- and y-axes (respectively) is equal to the mean ± (Range * I) 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 in this box.

Mark Selected Subsets. Click this button to display the Specify Multiple Subsets dialog, in which you can specify selection conditions that will subset (i.e., categorize) the cases in one plot. For more information about this option, see Mark Selected Subsets.

Bag plot properties. The options under Bag plot properties control the appearance of the bag plot.

Whisker type. There are three options available with the Whisker type: Off, All whiskers, and One per edge. Off suppresses the display of the whiskers. The All whiskers option will connect all points that are outside of the bag and within the fence to an edge of the bag. The One per edge option will connect only the point furthest away from a respective edge of the bag.

Show points in bag. Select the Show points in bag check box to display data points that fall within the bag.

Show outliers. Select the Show outliers check box to display the outliers, the data points that fall outside of the fence, with a unique point marker different than the point marker used for the raw data points.

Fence coeff. The fence coefficient is the factor that is used to magnify the bag to create the fence.

Bag coeff. The bag coefficient specifies the percentage of data points that the bag captures.