
SPREAD LOCATION PLOTName:
Given a response variable Y and a groupid variable X, the homoscedasticity plot is the group standard deviations versus the group means. This is a graphical measure of how the spread (variability) changes as the location changes across groups. This is basically a graphical test for the assumption of homogeneous variances across samples. The sl plot has the square roots of the absolute value of the Y(i) minus their group medians on the vertical axis and the group medians on the horizontal axis. A reference line connects the group medians. This provides a more robust measure of how the spread changes as the location changes. The basic idea can be applied to other measure of location and spread. That is, some measure of spread (or variability) is plotted on the vertical axis and some measure of location is plotted on the horizontal axis. Although Dataplot only supports the HOMOSCEDASTICITY PLOT and the SPREAD LOCATION PLOT directly, it should be straight forward to implement other alternatives as macros. When setting the LINE and CHARACTER commands, the reference line is the first trace and the data starts with trace 2 (each group is identified as a unique trace). That is, to draw the data points as circles and the reference line as a solid line, do something like the following
CHARACTER BLANK LINE BLANK ALL LINE SOLID SPREAD LOCATION PLOT Y X
<SUBSET/EXCEPT/FOR qualification> where <y> is a response variable; <tag> is a group identifier variable; and where the <SUBSET/EXCEPT/FOR qualification> is optional.
SPREAD LOCATION PLOT Y1 TAG SUBSET TAG > 2
 
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Date created: 6/5/2001 