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Dataplot Vol 1 Vol 2

SPREAD LOCATION PLOT

Name:
    SPREAD LOCATION PLOT
Type:
    Graphics Command
Purpose:
    Generates a spread location plot.
Description:
    The spread-location (s-l) plot is a robust alternative to the homoscedasticity plot.

    Given a response variable Y and a group-id 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 s-l 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 CIRCLE ALL
      CHARACTER BLANK
      LINE BLANK ALL
      LINE SOLID
      SPREAD LOCATION PLOT Y X
Syntax:
    SPREAD LOCATION PLOT <y> <tag>>
                            <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.
Examples:
    SPREAD LOCATION PLOT Y1 TAG
    SPREAD LOCATION PLOT Y1 TAG SUBSET TAG > 2
Default:
    None
Synonyms:
    None
Reference:
    "Visualizing Data", Cleveland, William S., Hobart Press, 1993.
Related Commands:
    LINES = Sets the type for plot lines.
    CHARACTER = Sets the type for plot characters.
    HOMOSCEDATICITY PLOT = Generates a homoscedasticity plot.
    BOX PLOT = Generates a box plot.
    PLOT = Generates a data or function plot.
    LEVENE TEST = Perform Levenes test for homogeneity of variances.
    BARTLET TEST = Perform Bartlet;s test for homogeneity of variances.
Applications:
    Exploratory Data Analysis
Implementation Date:
    2000/1
Program:
    SKIP 25
    READ SPLETT2.DAT Y MACHINE
    TIC OFFSET UNITS DATA
    XTIC OFFSET 0.5 0.5
    YTIC OFFSET 0.2 0.2
    YTIC MARK DECIMAL 1
    CHARACTER BLANK 1 2 3 4
    LINE SOLID BLANK BLANK BLANK BLANK
    TITLE AUTOMATIC
    Y1LABEL SPREAD
    X1LABEL LOCATION
    SPREAD LOCATION PLOT Y MACHINE

    plot generated by sample program

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Date created: 6/5/2001
Last updated: 10/19/2015

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