Variable Specification (PROBIT command)

The variable specification on PROBIT identifies the variables for response count, observation count, groups, and predictors. The variable specification is required.

• The variables must be specified first. The specification must include the response-count variable, followed by the keyword OF and then the observation-count variable.
• If the value of the response-count variable exceeds that of the observation-count variable, a procedure error occurs and PROBIT is not executed.
• At least one predictor (covariate) must be specified following the keyword WITH. The number of predictors is limited only by available workspace. All predictors must be continuous variables.
• You can specify a grouping variable (factor) after the keyword BY. Only one variable can be specified. It must be numeric and can contain only integer values. You must specify, in parentheses, a range indicating the minimum and maximum values for the grouping variable. Each integer value in the specified range defines a group.
• Cases with values for the grouping variable that are outside the specified range are excluded from the analysis.
• Keywords BY and WITH can appear in either order. However, both must follow the response-and-observation-count variables.

Example

PROBIT R OF N WITH X.

• The number of observations having the measured response appears in variable R, and the total number of observations is in N. The predictor is X.

Example

PROBIT  R OF N BY ROOT(1,2) WITH X.

PROBIT  R OF N WITH X BY ROOT(1,2).

• Because keywords BY and WITH can be used in either order, these two commands are equivalent. Each command specifies X as a continuous predictor and ROOT as a categorical grouping variable.
• Groups are identified by the levels of variable ROOT, which may be 1 or 2.
• For each combination of predictor and grouping variables, the variable R contains the number of observations with the response of interest, and N contains the total number of observations.