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`base_models`

  • Available in: Stacked Ensembles
  • Hyperparameter: no

Description​

H2O's Stacked Ensemble method is a supervised ensemble machine learning algorithm that finds the optimal combination of a collection of prediction algorithms using a process called stacking (or Super Learning). The algorithm that learns the optimal combination of the base learners is called the metalearning algorithm or metalearner.

The base_models parameter is used to specify a list of models (or model IDs) that can be stacked together. Models must have been cross-validated (that is, nfolds>1 or fold_column was specified), they all must use the same cross-validation folds, and keep_cross_validation_predictions must have been set to True. One way to guarantee identical folds across base models is to set fold_assignment = "Modulo" in all the base models. It is also possible to get identical folds by setting fold_assignment = "Random" when the same seed is used in all base models.

  • None

Example​

library(h2o)
h2o.init()

# import the higgs_train_5k train and test datasets
train <- h2o.importFile("https://s3.amazonaws.com/h2o-public-test-data/smalldata/testng/higgs_train_5k.csv")
test <- h2o.importFile("https://s3.amazonaws.com/h2o-public-test-data/smalldata/testng/higgs_test_5k.csv")

# Identify predictors and response
y <- "response"
x <- setdiff(names(train), y)

# Convert the response column in train and test datasets to a factor    
train[, y] <- as.factor(train[, y])
test[, y] <- as.factor(test[, y])


# Set number of folds for base learners   
nfolds <- 3  

# Train & Cross-validate a GBM model
my_gbm <- h2o.gbm(x = x,
                  y = y,
                  training_frame = train,
                  distribution = "bernoulli",
                  ntrees = 10,
                  nfolds = nfolds,
                  keep_cross_validation_predictions = TRUE,
                  seed = 1)

# Train & Cross-validate an RF model
my_rf <- h2o.randomForest(x = x,
                          y = y,
                          training_frame = train,
                          ntrees = 10,
                          nfolds = nfolds,
                          keep_cross_validation_predictions = TRUE,
                          seed = 1)


# Next we can train a few different ensembles using different metalearners

# Train a stacked ensemble using the default metalearner algorithm
stack <- h2o.stackedEnsemble(x = x,
                             y = y,
                             training_frame = train,
                             base_models = list(my_gbm, my_rf))
h2o.auc(h2o.performance(stack, test))
# 0.7570171

# Train a stacked ensemble using GBM as the metalearner algorithm
# The metalearner will use GBM default values
stack_gbm <- h2o.stackedEnsemble(x = x,
                                 y = y,
                                 training_frame = train,
                                 base_models = list(my_gbm, my_rf),
                                 metalearner_algorithm = "gbm")
h2o.auc(h2o.performance(stack_gbm, test))
# 0.7511055                                 

# Train a stacked ensemble using RF as the metalearner algorithm
# The metelearner will use RF default values
stack_rf <- h2o.stackedEnsemble(x = x,
                                y = y,
                                training_frame = train,
                                base_models = list(my_gbm, my_rf),
                                metalearner_algorithm = "drf")
h2o.auc(h2o.performance(stack_rf, test))
# 0.7232461

# Train a stacked ensemble using Deep Learning as the metalearner algorithm
# The metelearner will use RF default values
stack_dl <- h2o.stackedEnsemble(x = x,
                                y = y,
                                training_frame = train,
                                base_models = list(my_gbm, my_rf),
                                metalearner_algorithm = "deeplearning")
h2o.auc(h2o.performance(stack_dl, test))
# 0.7571556
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