# Model Selection (linear models)

Model selection method. Choose one of the model selection methods (details below) or Include all predictors, which simply enters all available predictors as main effects model terms. By default, Forward stepwise is used.

Forward Stepwise Selection. This starts with no effects in the model and adds and removes effects one step at a time until no more can be added or removed according to the stepwise criteria.

- Criteria for entry/removal. This is the statistic used to
determine whether an effect should be added to or removed from the model. Information
Criterion (AICC) is based on the likelihood of the training set given the model, and is
adjusted to penalize overly complex models. F Statistics is based on a
statistical test of the improvement in model error. Adjusted R-squared is
based on the fit of the training set, and is adjusted to penalize overly complex models.
Overfit Prevention Criterion (ASE) is based on the fit (average squared
error, or ASE) of the overfit prevention set. The overfit prevention set is a random subsample of
approximately 30% of the original dataset that is not used to train the model.
If any criterion other than F Statistics is chosen, then at each step the effect that corresponds to the greatest positive increase in the criterion is added to the model. Any effects in the model that correspond to a decrease in the criterion are removed.

If F Statistics is chosen as the criterion, then at each step the effect that has the smallest

*p*-value less than the specified threshold, Include effects with p-values less than, is added to the model. The default is 0.05. Any effects in the model with a*p*-value greater than the specified threshold, Remove effects with p-values greater than, are removed. The default is 0.10. - Customize maximum number of effects in the final model. By default, all available effects can be entered into the model. Alternatively, if the stepwise algorithm ends a step with the specified maximum number of effects, the algorithm stops with the current set of effects.
- Customize maximum number of steps. The stepwise algorithm stops after a certain number of steps. By default, this is 3 times the number of available effects. Alternatively, specify a positive integer maximum number of steps.

Best Subsets Selection. This checks "all possible" models, or at least a larger subset of the possible models than forward stepwise, to choose the best according to the best subsets criterion. Information Criterion (AICC) is based on the likelihood of the training set given the model, and is adjusted to penalize overly complex models. Adjusted R-squared is based on the fit of the training set, and is adjusted to penalize overly complex models. Overfit Prevention Criterion (ASE) is based on the fit (average squared error, or ASE) of the overfit prevention set. The overfit prevention set is a random subsample of approximately 30% of the original dataset that is not used to train the model.

The model with the greatest value of the criterion is chosen as the best model.