# econml.policy.PolicyForest

class econml.policy.PolicyForest(n_estimators=100, *, criterion='neg_welfare', max_depth=None, min_samples_split=10, min_samples_leaf=5, min_weight_fraction_leaf=0.0, max_features='auto', min_impurity_decrease=0.0, max_samples=0.5, min_balancedness_tol=0.45, honest=True, n_jobs=- 1, random_state=None, verbose=0, warm_start=False)[source]

Bases: econml._ensemble._ensemble.BaseEnsemble

Welfare maximization policy forest. Trains a forest to maximize the objective: $$1/n \sum_i \sum_j a_j(X_i) * y_{ij}$$, where, where $$a(X)$$ is constrained to take value of 1 only on one coordinate and zero otherwise. This corresponds to a policy optimization problem.

Parameters
• n_estimators (integer, optional (default=100)) – The total number of trees in the forest. The forest consists of a forest of sqrt(n_estimators) sub-forests, where each sub-forest contains sqrt(n_estimators) trees.

• criterion ({'neg_welfare'}, default=’neg_welfare’) – The criterion type

• max_depth (int, default=None) – The maximum depth of the tree. If None, then nodes are expanded until all leaves are pure or until all leaves contain less than min_samples_split samples.

• min_samples_split (int or float, default=10) – The minimum number of samples required to split an internal node:

• If int, then consider min_samples_split as the minimum number.

• If float, then min_samples_split is a fraction and ceil(min_samples_split * n_samples) are the minimum number of samples for each split.

• min_samples_leaf (int or float, default=5) – The minimum number of samples required to be at a leaf node. A split point at any depth will only be considered if it leaves at least min_samples_leaf training samples in each of the left and right branches. This may have the effect of smoothing the model, especially in regression.

• If int, then consider min_samples_leaf as the minimum number.

• If float, then min_samples_leaf is a fraction and ceil(min_samples_leaf * n_samples) are the minimum number of samples for each node.

• min_weight_fraction_leaf (float, default=0.0) – The minimum weighted fraction of the sum total of weights (of all the input samples) required to be at a leaf node. Samples have equal weight when sample_weight is not provided.

• max_features (int, float or {“auto”, “sqrt”, “log2”}, default=None) – The number of features to consider when looking for the best split:

• If int, then consider max_features features at each split.

• If float, then max_features is a fraction and int(max_features * n_features) features are considered at each split.

• If “auto”, then max_features=n_features.

• If “sqrt”, then max_features=sqrt(n_features).

• If “log2”, then max_features=log2(n_features).

• If None, then max_features=n_features.

Note: the search for a split does not stop until at least one valid partition of the node samples is found, even if it requires to effectively inspect more than max_features features.

• min_impurity_decrease (float, default=0.0) – A node will be split if this split induces a decrease of the impurity greater than or equal to this value. The weighted impurity decrease equation is the following:

N_t / N * (impurity - N_t_R / N_t * right_impurity
- N_t_L / N_t * left_impurity)


where N is the total number of samples, N_t is the number of samples at the current node, N_t_L is the number of samples in the left child, and N_t_R is the number of samples in the right child. N, N_t, N_t_R and N_t_L all refer to the weighted sum, if sample_weight is passed.

• max_samples (int or float in (0, 1], default=.5,) – The number of samples to use for each subsample that is used to train each tree:

• If int, then train each tree on max_samples samples, sampled without replacement from all the samples

• If float, then train each tree on ceil(max_samples * n_samples), sampled without replacement from all the samples.

• min_balancedness_tol (float in [0, .5], default=.45) – How imbalanced a split we can tolerate. This enforces that each split leaves at least (.5 - min_balancedness_tol) fraction of samples on each side of the split; or fraction of the total weight of samples, when sample_weight is not None. Default value, ensures that at least 5% of the parent node weight falls in each side of the split. Set it to 0.0 for no balancedness and to .5 for perfectly balanced splits. For the formal inference theory to be valid, this has to be any positive constant bounded away from zero.

• honest (bool, default=True) – Whether the data should be split in two equally sized samples, such that the one half-sample is used to determine the optimal split at each node and the other sample is used to determine the value of every node.

• n_jobs (int or None, optional (default=-1)) – The number of jobs to run in parallel for both fit and predict. None means 1 unless in a joblib.parallel_backend() context. -1 means using all processors. See Glossary for more details.

• verbose (int, optional (default=0)) – Controls the verbosity when fitting and predicting.

• random_state (int, RandomState instance or None, optional (default=None)) – If int, random_state is the seed used by the random number generator; If RandomState instance, random_state is the random number generator; If None, the random number generator is the RandomState instance used by np.random.

• warm_start (bool, default=False) – When set to True, reuse the solution of the previous call to fit and add more estimators to the ensemble, otherwise, just fit a whole new forest.

feature_importances_

The feature importances based on the amount of parameter heterogeneity they create. The higher, the more important the feature.

Type

ndarray of shape (n_features,)

__init__(n_estimators=100, *, criterion='neg_welfare', max_depth=None, min_samples_split=10, min_samples_leaf=5, min_weight_fraction_leaf=0.0, max_features='auto', min_impurity_decrease=0.0, max_samples=0.5, min_balancedness_tol=0.45, honest=True, n_jobs=- 1, random_state=None, verbose=0, warm_start=False)[source]

Methods

 __init__([n_estimators, criterion, ...]) Apply trees in the forest to X, return leaf indices. Return the decision path in the forest. feature_importances([max_depth, ...]) The feature importances based on the amount of parameter heterogeneity they create. fit(X, y, *[, sample_weight]) Build a forest of trees from the training set (X, y) and any other auxiliary variables. get_params([deep]) Get parameters for this estimator. Re-generate the example same sample indices as those at fit time using same pseudo-randomness. Predict the best treatment for each sample Predict the probability of recommending each treatment Predict the expected value of each treatment for each sample set_params(**params) Set the parameters of this estimator.

Attributes

apply(X)[source]

Apply trees in the forest to X, return leaf indices.

Parameters

X (array-like of shape (n_samples, n_features)) – The input samples. Internally, it will be converted to dtype=np.float64.

Returns

X_leaves – For each datapoint x in X and for each tree in the forest, return the index of the leaf x ends up in.

Return type

ndarray of shape (n_samples, n_estimators)

decision_path(X)[source]

Return the decision path in the forest.

Parameters

X (array-like of shape (n_samples, n_features)) – The input samples. Internally, it will be converted to dtype=np.float64.

Returns

• indicator (sparse matrix of shape (n_samples, n_nodes)) – Return a node indicator matrix where non zero elements indicates that the samples goes through the nodes. The matrix is of CSR format.

• n_nodes_ptr (ndarray of shape (n_estimators + 1,)) – The columns from indicator[n_nodes_ptr[i]:n_nodes_ptr[i+1]] gives the indicator value for the i-th estimator.

feature_importances(max_depth=4, depth_decay_exponent=2.0)[source]

The feature importances based on the amount of parameter heterogeneity they create. The higher, the more important the feature.

Parameters
• max_depth (int, default=4) – Splits of depth larger than max_depth are not used in this calculation

• depth_decay_exponent (double, default=2.0) – The contribution of each split to the total score is re-weighted by 1 / (1 + depth)**2.0.

Returns

feature_importances_ – Normalized total parameter heterogeneity inducing importance of each feature

Return type

ndarray of shape (n_features,)

fit(X, y, *, sample_weight=None, **kwargs)[source]

Build a forest of trees from the training set (X, y) and any other auxiliary variables.

Parameters
• X (array-like of shape (n_samples, n_features)) – The training input samples. Internally, its dtype will be converted to dtype=np.float64.

• y (array-like of shape (n_samples,) or (n_samples, n_treatments)) – The outcome values for each sample and for each treatment.

• sample_weight (array-like of shape (n_samples,), default=None) – Sample weights. If None, then samples are equally weighted. Splits that would create child nodes with net zero or negative weight are ignored while searching for a split in each node.

• **kwargs (dictionary of array-like items of shape (n_samples, d_var)) – Auxiliary random variables

Returns

self

Return type

object

get_params(deep=True)

Get parameters for this estimator.

Parameters

deep (bool, default=True) – If True, will return the parameters for this estimator and contained subobjects that are estimators.

Returns

params – Parameter names mapped to their values.

Return type

dict

get_subsample_inds()[source]

Re-generate the example same sample indices as those at fit time using same pseudo-randomness.

predict(X)[source]

Predict the best treatment for each sample

Parameters

X ({array-like} of shape (n_samples, n_features)) – The input samples. Internally, it will be converted to dtype=np.float64.

Returns

treatment – The recommded treatment, i.e. the treatment index most often predicted to have the highest reward for each sample. Recommended treatments are aggregated from each tree in the ensemble and the treatment that receives the most votes is returned. Use predict_proba to get the fraction of trees in the ensemble that recommend each treatment for each sample.

Return type

array-like of shape (n_samples)

predict_proba(X)[source]

Predict the probability of recommending each treatment

Parameters
• X ({array-like} of shape (n_samples, n_features)) – The input samples. Internally, it will be converted to dtype=np.float64.

• check_input (bool, default=True) – Allow to bypass several input checking. Don’t use this parameter unless you know what you do.

Returns

treatment_proba – The probability of each treatment recommendation

Return type

array-like of shape (n_samples, n_treatments)

predict_value(X)[source]

Predict the expected value of each treatment for each sample

Parameters

X ({array-like} of shape (n_samples, n_features)) – The input samples. Internally, it will be converted to dtype=np.float64.

Returns

welfare – The conditional average welfare for each treatment for the group of each sample defined by the tree

Return type

array-like of shape (n_samples, n_treatments)

set_params(**params)

Set the parameters of this estimator.

The method works on simple estimators as well as on nested objects (such as Pipeline). The latter have parameters of the form <component>__<parameter> so that it’s possible to update each component of a nested object.

Parameters

**params (dict) – Estimator parameters.

Returns

self – Estimator instance.

Return type

estimator instance