# Model Stacking

In a model stack, as introduced by Wolpert (1992), an adjudicating model learns the best way to combine the predictions of multiple base models. In MLJ, such models are constructed using the Stack constructor. To learn more about stacking and to see how to construct a stack "by hand" using Learning Networks, see this Data Science in Julia tutorial

MLJBase.StackType
Stack(; metalearner=nothing, name1=model1, name2=model2, ..., keyword_options...)

Implements the two-layer generalized stack algorithm introduced by Wolpert (1992) and generalized by Van der Laan et al (2007). Returns an instance of type ProbablisiticStack or DeterministicStack, depending on the prediction type of metalearner.

When training a machine bound to such an instance:

• The data is split into training/validation sets according to the specified resampling strategy.

• Each base model model1, model2, ... is trained on each training subset and outputs predictions on the corresponding validation sets. The multi-fold predictions are spliced together into a so-called out-of-sample prediction for each model.

• The adjudicating model, metalearner, is subsequently trained on the out-of-sample predictions to learn the best combination of base model predictions.

• Each base model is retrained on all supplied data for purposes of passing on new production data onto the adjudicator for making new predictions

Arguments

• metalearner::Supervised: The model that will optimize the desired criterion based on its internals. For instance, a LinearRegression model will optimize the squared error.

• resampling: The resampling strategy used to prepare out-of-sample predictions of the base learners.

• measures: A measure or iterable over measures, to perform an internal evaluation of the learners in the Stack while training. This is not for the evaluation of the Stack itself.

• cache: Whether machines created in the learning network will cache data or not.

• acceleration: A supported AbstractResource to define the training parallelization mode of the stack.

• name1=model1, name2=model2, ...: the Supervised model instances to be used as base learners. The provided names become properties of the instance created to allow hyper-parameter access

Example

The following code defines a DeterministicStack instance for learning a Continuous target, and demonstrates that:

• Base models can be Probabilistic models even if the stack itself is Deterministic (predict_mean is applied in such cases).

• As an alternative to hyperparameter optimization, one can stack multiple copies of given model, mutating the hyper-parameter used in each copy.

using MLJ

X, y = make_regression(500, 5)

stack = Stack(;metalearner=LinearRegressor(),
resampling=CV(),
measures=rmse,
constant=ConstantRegressor(),
tree_2=DecisionTreeRegressor(max_depth=2),
tree_3=DecisionTreeRegressor(max_depth=3),
evo=EvoTreeRegressor(),
knn=KNNRegressor(),
xgb=XGBoostRegressor())

mach = machine(stack, X, y)
evaluate!(mach; resampling=Holdout(), measure=rmse)


The internal evaluation report can be accessed like this and provides a PerformanceEvaluation object for each model:

report(mach).cv_report