Inclusion Levels (DISCRIMINANT command)

When you specify a stepwise method on the METHOD subcommand (any method other than the default direct-entry method), you can control the order in which variables are considered for entry or removal by specifying inclusion levels on the ANALYSIS subcommand. By default, all variables in the analysis are entered according to the criterion requested on the METHOD subcommand.

  • An inclusion level is an integer between 0 and 99, specified in parentheses after a variable or list of variables on the ANALYSIS subcommand.
  • The default inclusion level is 1.
  • Variables with higher inclusion levels are considered for entry before variables with lower inclusion levels.
  • Variables with even inclusion levels are entered as a group.
  • Variables with odd inclusion levels are entered individually, according to the stepwise method specified on the METHOD subcommand.
  • Only variables with an inclusion level of 1 are considered for removal. To make a variable with a higher inclusion level eligible for removal, name it twice on the ANALYSIS subcommand, first specifying the desired inclusion level and then an inclusion level of 1.
  • Variables with an inclusion level of 0 are never entered. However, the statistical criterion for entry is computed and displayed.
  • Variables that fail the tolerance criterion are not entered regardless of their inclusion level.

The following are some common methods of entering variables and the inclusion levels that could be used to achieve them. These examples assume that one of the stepwise methods is specified on the METHOD subcommand (otherwise, inclusion levels have no effect).

Direct. ANALYSIS=ALL(2) forces all variables into the equation. (This is the default and can be requested with METHOD=DIRECT or simply by omitting the METHOD subcommand.)

Stepwise. ANALYSIS=ALL(1) yields a stepwise solution in which variables are entered and removed in stepwise fashion. (This is the default when anything other than DIRECT is specified on the METHOD subcommand.)

Forward. ANALYSIS=ALL(3) enters variables into the equation stepwise but does not remove variables.

Backward. ANALYSIS=ALL(2) ALL(1) forces all variables into the equation and then allows them to be removed stepwise if they satisfy the criterion for removal.

Inclusion Levels Used With a Stepwise Method

DISCRIMINANT GROUPS=SUCCESS(0,1)
  /VARIABLES=A, B, C, D, E
  /ANALYSIS=A TO C (2) D, E (1)
  /METHOD=WILKS.
  • A, B, and C are entered into the analysis first, assuming that they pass the tolerance criterion. Since their inclusion level is even, they are entered together.
  • D and E are then entered stepwise. The one that minimizes the overall value of Wilks’ lambda is entered first.
  • After entering D and E, the program checks whether the partial F for either one justifies removal from the equation (see the FOUT and POUT subcommands).

Inclusion Levels Without a Stepwise Method

DISCRIMINANT GROUPS=SUCCESS(0,1)
  /VARIABLES=A, B, C, D, E
  /ANALYSIS=A TO C (2) D, E (1).
  • Since no stepwise method is specified, inclusion levels have no effect and all variables are entered into the model at once.