15.14 ONEWAY

ONEWAY
        [/VARIABLES = ] var_list BY var
        /MISSING={ANALYSIS,LISTWISE} {EXCLUDE,INCLUDE}
        /CONTRAST= value1 [, value2] ... [,valueN]
        /STATISTICS={DESCRIPTIVES,HOMOGENEITY}
        /POSTHOC={BONFERRONI, GH, LSD, SCHEFFE, SIDAK, TUKEY, ALPHA ([value])}

The ONEWAY procedure performs a one-way analysis of variance of variables factored by a single independent variable. It is used to compare the means of a population divided into more than two groups.

The dependent variables to be analysed should be given in the VARIABLES subcommand. The list of variables must be followed by the BY keyword and the name of the independent (or factor) variable.

You can use the STATISTICS subcommand to tell PSPP to display ancillary information. The options accepted are:

• DESCRIPTIVES Displays descriptive statistics about the groups factored by the independent variable.
• HOMOGENEITY Displays the Levene test of Homogeneity of Variance for the variables and their groups.

The CONTRAST subcommand is used when you anticipate certain differences between the groups. The subcommand must be followed by a list of numerals which are the coefficients of the groups to be tested. The number of coefficients must correspond to the number of distinct groups (or values of the independent variable). If the total sum of the coefficients are not zero, then PSPP will display a warning, but will proceed with the analysis. The CONTRAST subcommand may be given up to 10 times in order to specify different contrast tests. The MISSING subcommand defines how missing values are handled. If LISTWISE is specified then cases which have missing values for the independent variable or any dependent variable are ignored. If ANALYSIS is specified, then cases are ignored if the independent variable is missing or if the dependent variable currently being analysed is missing. The default is ANALYSIS. A setting of EXCLUDE means that variables whose values are user-missing are to be excluded from the analysis. A setting of INCLUDE means they are to be included. The default is EXCLUDE.

Using the POSTHOC subcommand you can perform multiple pairwise comparisons on the data. The following comparison methods are available:

• LSD Least Significant Difference.
• TUKEY Tukey Honestly Significant Difference.
• BONFERRONI Bonferroni test.
• SCHEFFE Scheffé’s test.
• SIDAK Sidak test.
• GH The Games-Howell test.

Use the optional syntax ALPHA(value) to indicate that ONEWAY should perform the posthoc tests at a confidence level of value. If ALPHA(value) is not specified, then the confidence level used is 0.05.