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%matplotlib inline


# Model-based and sequential feature selection¶

This example illustrates and compares two approaches for feature selection: :class:~sklearn.feature_selection.SelectFromModel which is based on feature importance, and :class:~sklearn.feature_selection.SequentialFeatureSelection which relies on a greedy approach.

We use the Diabetes dataset, which consists of 10 features collected from 442 diabetes patients.

Authors: Manoj Kumar <[email protected]>, Maria Telenczuk <https://github.com/maikia>, Nicolas Hug.

In [ ]:
print(__doc__)


We first load the diabetes dataset which is available from within scikit-learn, and print its description:

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from sklearn.datasets import load_diabetes

X, y = diabetes.data, diabetes.target
print(diabetes.DESCR)


## Feature importance from coefficients¶

To get an idea of the importance of the features, we are going to use the :class:~sklearn.linear_model.LassoCV estimator. The features with the highest absolute coef_ value are considered the most important. We can observe the coefficients directly without needing to scale them (or scale the data) because from the description above, we know that the features were already standardized. For a more complete example on the interpretations of the coefficients of linear models, you may refer to sphx_glr_auto_examples_inspection_plot_linear_model_coefficient_interpretation.py.

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import matplotlib.pyplot as plt
import numpy as np
from sklearn.linear_model import LassoCV

lasso = LassoCV().fit(X, y)
importance = np.abs(lasso.coef_)
feature_names = np.array(diabetes.feature_names)
plt.bar(height=importance, x=feature_names)
plt.title("Feature importances via coefficients")
plt.show()


## Selecting features based on importance¶

Now we want to select the two features which are the most important according to the coefficients. The :class:~sklearn.feature_selection.SelectFromModel is meant just for that. :class:~sklearn.feature_selection.SelectFromModel accepts a threshold parameter and will select the features whose importance (defined by the coefficients) are above this threshold.

Since we want to select only 2 features, we will set this threshold slightly above the coefficient of third most important feature.

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from sklearn.feature_selection import SelectFromModel
from time import time

threshold = np.sort(importance)[-3] + 0.01

tic = time()
sfm = SelectFromModel(lasso, threshold=threshold).fit(X, y)
toc = time()
print("Features selected by SelectFromModel: "
f"{feature_names[sfm.get_support()]}")
print(f"Done in {toc - tic:.3f}s")


## Selecting features with Sequential Feature Selection¶

Another way of selecting features is to use :class:~sklearn.feature_selection.SequentialFeatureSelector (SFS). SFS is a greedy procedure where, at each iteration, we choose the best new feature to add to our selected features based a cross-validation score. That is, we start with 0 features and choose the best single feature with the highest score. The procedure is repeated until we reach the desired number of selected features.

We can also go in the reverse direction (backward SFS), i.e. start with all the features and greedily choose features to remove one by one. We illustrate both approaches here.

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from sklearn.feature_selection import SequentialFeatureSelector

tic_fwd = time()
sfs_forward = SequentialFeatureSelector(lasso, n_features_to_select=2,
direction='forward').fit(X, y)
toc_fwd = time()

tic_bwd = time()
sfs_backward = SequentialFeatureSelector(lasso, n_features_to_select=2,
direction='backward').fit(X, y)
toc_bwd = time()

print("Features selected by forward sequential selection: "
f"{feature_names[sfs_forward.get_support()]}")
print(f"Done in {toc_fwd - tic_fwd:.3f}s")
print("Features selected by backward sequential selection: "
f"{feature_names[sfs_backward.get_support()]}")
print(f"Done in {toc_bwd - tic_bwd:.3f}s")


## Discussion¶

Interestingly, forward and backward selection have selected the same set of features. In general, this isn't the case and the two methods would lead to different results.

We also note that the features selected by SFS differ from those selected by feature importance: SFS selects bmi instead of s1. This does sound reasonable though, since bmi corresponds to the third most important feature according to the coefficients. It is quite remarkable considering that SFS makes no use of the coefficients at all.

To finish with, we should note that :class:~sklearn.feature_selection.SelectFromModel is significantly faster than SFS. Indeed, :class:~sklearn.feature_selection.SelectFromModel only needs to fit a model once, while SFS needs to cross-validate many different models for each of the iterations. SFS however works with any model, while :class:~sklearn.feature_selection.SelectFromModel requires the underlying estimator to expose a coef_ attribute or a feature_importances_ attribute. The forward SFS is faster than the backward SFS because it only needs to perform n_features_to_select = 2 iterations, while the backward SFS needs to perform n_features - n_features_to_select = 8 iterations.