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The `GROUPS`

subcommand invokes Independent Samples mode or
‘Groups’ mode.
This mode is used to test whether two groups of values have the
same population mean.
In this mode, you must also use the `/VARIABLES`

subcommand to
tell PSPP the dependent variables you wish to test.

The variable given in the `GROUPS`

subcommand is the independent
variable which determines to which group the samples belong.
The values in parentheses are the specific values of the independent
variable for each group.
If the parentheses are omitted and no values are given, the default values
of 1.0 and 2.0 are assumed.

If the independent variable is numeric,
it is acceptable to specify only one value inside the parentheses.
If you do this, cases where the independent variable is
greater than or equal to this value belong to the first group, and cases
less than this value belong to the second group.
When using this form of the `GROUPS`

subcommand, missing values in
the independent variable are excluded on a listwise basis, regardless
of whether `/MISSING=LISTWISE`

was specified.