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ML | Chi-square Test for feature selection

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Feature selection is also known as attribute selection is a process of extracting the most relevant features from the dataset and then applying machine learning algorithms for the better performance of the model. A large number of irrelevant features increases the training time exponentially and increase the risk of overfitting.

Chi-square Test for Feature Extraction:
Chi-square test is used for categorical features in a dataset. We calculate Chi-square between each feature and the target and select the desired number of features with best Chi-square scores. It determines if the association between two categorical variables of the sample would reflect their real association in the population.
Chi- square score is given by :

where –

Observed frequency = No. of observations of class
Expected frequency = No. of expected observations of class if there was no relationship between the feature and the target.

Python Implementation of Chi-Square feature selection:

# Load libraries
from sklearn.datasets import load_iris
from sklearn.feature_selection import SelectKBest
from sklearn.feature_selection import chi2
# Load iris data
iris_dataset = load_iris()
# Create features and target
X =
y =
# Convert to categorical data by converting data to integers
X = X.astype(int)
# Two features with highest chi-squared statistics are selected
chi2_features = SelectKBest(chi2, k = 2)
X_kbest_features = chi2_features.fit_transform(X, y)
# Reduced features
print('Original feature number:', X.shape[1])
print('Reduced feature number:', X_kbest.shape[1])


Original feature number: 4
Reduced feature number : 2
Last Updated : 24 Dec, 2018
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