Modeling and Multivariate Methods - SAS

206 Performing Logistic Regression on Nominal and Ordinal Responses Chapter 7
Logistic Fit Platform Options
Logistic Fit Platform Options
Plot Options
The red triangle menu next to the analysis name gives you the additional options that are described next.
These options are described in the Basic Analysis and Graphing book.
Likelihood Ratio Tests
See “Likelihood Ratio Tests” on page 205.
Wald Tests for Effects
One downside to likelihood ratio tests is that they involve refitting the whole model, which uses another
series of iterations. Therefore, they could take a long time for big problems. The logistic fitting platform
gives an optional test, which is more straightforward, serving the same function. The Wald Chi-square is a
quadratic approximation to the likelihood-ratio test, and it is a by-product of the calculations. Though
Wald tests are considered less trustworthy, they do provide an adequate significance indicator for screening
effects. Each parameter estimate and effect is shown with a Wald test. This is the default test if the fit takes
more than ten seconds to complete.
Figure 7.7 Effect Wald Tests
Likelihood-ratio tests are the platform default and are discussed under “Likelihood Ratio Tests” on page 205.
It is highly recommended to use this default option.
Confidence Intervals
You can also request profile likelihood confidence intervals for the model parameters. When you select the
Confidence Intervals command, a dialog prompts you to enter α to compute the 1 – α confidence
intervals, or you can use the default of α = 0.05. Each confidence limit requires a set of iterations in the
model fit and can be expensive. Furthermore, the effort does not always succeed in finding limits.