EXACT
<’label’> <INTERCEPT> <effects> </ options> ;
The EXACT statement performs exact tests of the parameters for the specified effects and optionally estimates the parameters and outputs the exact conditional distributions. You can specify the keyword INTERCEPT and any effects in the MODEL statement. Inference on the parameters of the specified effects is performed by conditioning on the sufficient statistics of all the other model parameters (possibly including the intercept).
You can specify several EXACT statements, but they must follow the MODEL statement. Each statement can optionally include an identifying label. If several EXACT statements are specified, any statement without a label is assigned a label of the form “Exactn,” where n indicates the nth EXACT statement. The label is included in the headers of the displayed exact analysis tables.
If a STRATA statement is also specified, then a stratified exact logistic regression is performed. The model contains a different intercept for each stratum, and these intercepts are conditioned out of the model along with any other nuisance parameters (parameters for effects specified in the MODEL statement that are not in the EXACT statement).
If the LINK=GLOGIT option is specified in the MODEL statement, then the METHOD=DIRECT option is invoked in the EXACTOPTIONS statement by default and a generalized logit model is fit. Since each effect specified in the MODEL statement adds k parameters to the model (where k + 1 is the number of response levels), exact analysis of the generalized logit model by using this method is limited to rather small problems.
The CONTRAST, ESTIMATE, LSMEANS, LSMESTIMATE, ODDSRATIO, OUTPUT, ROC, ROCCONTRAST, SCORE, SLICE, STORE, TEST, and UNITS statements are not available with an exact analysis; results from these statements are based on the asymptotic results. Exact analyses are not performed when you specify a WEIGHT statement, a link other than LINK=LOGIT or LINK=GLOGIT, an offset variable, the NOFIT option, or a model selection method. Exact estimation is not available for ordinal response models.
For classification variables, use of the reference parameterization is recommended.
The following options can be specified in each EXACT statement after a slash (/):
In the following example, two exact tests are computed: one for x1
and the other for x2
. The test for x1
is based on the exact conditional distribution of the sufficient statistic for the x1
parameter given the observed values of the sufficient statistics for the intercept, x2
, and x3
parameters; likewise, the test for x2
is conditional on the observed sufficient statistics for the intercept, x1
, and x3
.
proc logistic; model y= x1 x2 x3; exact x1 x2; run;
PROC LOGISTIC determines, from all the specified EXACT statements, the distinct conditional distributions that need to be evaluated. For example, there is only one exact conditional distribution for the following two EXACT statements:
exact 'One' x1 / estimate=parm; exact 'Two' x1 / estimate=parm onesided;
For each EXACT statement, individual tests for the parameters of the specified effects are computed unless the JOINTONLY option is specified. Consider the following EXACT statements:
exact 'E12' x1 x2 / estimate; exact 'E1' x1 / estimate; exact 'E2' x2 / estimate; exact 'J12' x1 x2 / joint;
In the E12 statement, the parameters for x1
and x2
are estimated and tested separately. Specifying the E12 statement is equivalent to specifying both the E1 and E2 statements.
In the J12 statement, the joint test for the parameters of x1
and x2
is computed in addition to the individual tests for x1
and x2
.