Regardless of the criterion used (e.g. the default "Deviance" criterion, or the "AIC" criterion), the stepwise method brings in one variable at a time, but then makes a fit assessment of the whole model (not just that term):
- For deviance, the metric for the criterion is the p-value of the F test* (which is a whole-model test).
- For AIC, the metric is the AIC. (Again, a whole-model test.)
For details, see the Criterion section of the documentation for stepwiseglm. Also, look at the PEnter and PRemove parameters for the precise numeric values of the thresholds.
*Or chi-squared test
