Issue
I wrote a very simple multiclass classifier based on the iris dataset. This is the code:
import numpy as np
from sklearn.datasets import load_iris
from sklearn.model_selection import train_test_split
from sklearn.pipeline import make_pipeline
from sklearn.preprocessing import StandardScaler
from sklearn.svm import LinearSVC, SVC
from sklearn.preprocessing import label_binarize
from sklearn.multiclass import OneVsRestClassifier
from sklearn.linear_model import SGDClassifier
from sklearn.metrics import classification_report
# Load the data
iris = load_iris()
X = iris.data
y = iris.target
# Use label_binarize to be multi-label like settings
Y = label_binarize(y, classes=[0, 1, 2])
n_classes = Y.shape[1]
# Add noisy features
random_state = np.random.RandomState(0)
n_samples, n_features = X.shape
X = np.concatenate([X, random_state.randn(n_samples, 200 * n_features)], axis=1)
from sklearn.preprocessing import label_binarize
# Split into training and test
X_train, X_test, y_train, y_test = train_test_split(
X, Y, test_size=0.5, random_state=0
)
# Create classifier
classifier = OneVsRestClassifier(
make_pipeline(StandardScaler(), LinearSVC(random_state=random_state))
)
# Train the model
classifier.fit(X_train, y_train)
My goal is to predict the values of the test set in 2 ways:
- Using the
classifier.predict()function and definey_pred. - Using the
classifier.decision_function()to get the scores and then pick the highest one for each instance and definey_pred_.
Here is how I did it:
# Get the scores for the Test set
y_score = classifier.decision_function(X_test)
# Make predictions
y_pred = classifier.predict(X_test)
y_pred_ = label_binarize(np.argmax(y_score, axis=1), [0,1,2])
It looks like however that when I try to compute the classification report I get slightly different results, while I would expect to be the same since the predictions are based on the scores obtained from the decision function as it can be seen in the documentation (line 789). Here are both reports:
print(classification_report(y_test, y_pred))
print(classification_report(y_test, y_pred_))
precision recall f1-score support
0 0.54 0.62 0.58 21
1 0.44 0.40 0.42 30
2 0.36 0.50 0.42 24
micro avg 0.44 0.49 0.47 75
macro avg 0.45 0.51 0.47 75
weighted avg 0.45 0.49 0.46 75
samples avg 0.39 0.49 0.42 75
precision recall f1-score support
0 0.42 0.38 0.40 21
1 0.52 0.47 0.49 30
2 0.38 0.46 0.42 24
micro avg 0.44 0.44 0.44 75
macro avg 0.44 0.44 0.44 75
weighted avg 0.45 0.44 0.44 75
samples avg 0.44 0.44 0.44 75
What am I doing wrong? Would you be able to suggest a smart and elegant solution so that both reports are identical?
Solution
For multilabel classification you should use
y_pred_ = np.where(classifier.decision_function(X_test) > 0, 1, 0)
to replicate the output of the predict() method as in this case the different classes are not mutually exclusive, i.e. a given sample can belong to multiple classes.
import numpy as np
from sklearn.datasets import load_iris
from sklearn.model_selection import train_test_split
from sklearn.pipeline import make_pipeline
from sklearn.preprocessing import StandardScaler, label_binarize
from sklearn.svm import LinearSVC
from sklearn.multiclass import OneVsRestClassifier
from sklearn.metrics import classification_report
# Load the data
iris = load_iris()
X = iris.data
y = label_binarize(iris.target, classes=[0, 1, 2])
# Split the data into training and test
X_train, X_test, y_train, y_test = train_test_split(
X, y, test_size=0.5, random_state=0
)
# Create classifier
classifier = OneVsRestClassifier(
make_pipeline(StandardScaler(), LinearSVC(random_state=0))
)
# Train the model
classifier.fit(X_train, y_train)
# Make predictions
y_pred = classifier.predict(X_test)
y_pred_ = np.where(classifier.decision_function(X_test) > 0, 1, 0)
print(classification_report(y_test, y_pred))
# precision recall f1-score support
# 0 1.00 1.00 1.00 21
# 1 0.58 0.37 0.45 30
# 2 0.95 0.83 0.89 24
# micro avg 0.85 0.69 0.76 75
# macro avg 0.84 0.73 0.78 75
# weighted avg 0.82 0.69 0.74 75
# samples avg 0.66 0.69 0.67 75
print(classification_report(y_test, y_pred_))
# precision recall f1-score support
# 0 1.00 1.00 1.00 21
# 1 0.58 0.37 0.45 30
# 2 0.95 0.83 0.89 24
# micro avg 0.85 0.69 0.76 75
# macro avg 0.84 0.73 0.78 75
# weighted avg 0.82 0.69 0.74 75
# samples avg 0.66 0.69 0.67 75
For multiclass classification you can instead use
y_pred_ = np.argmax(classifier.decision_function(X_test), axis=1)
as in your code, as in this case the different classes are mutually exclusive, i.e. each sample is assigned to only one class.
import numpy as np
from sklearn.datasets import load_iris
from sklearn.model_selection import train_test_split
from sklearn.pipeline import make_pipeline
from sklearn.preprocessing import StandardScaler
from sklearn.svm import LinearSVC
from sklearn.multiclass import OneVsRestClassifier
from sklearn.metrics import classification_report
# Load the data
iris = load_iris()
X = iris.data
y = iris.target
# Split into training and test
X_train, X_test, y_train, y_test = train_test_split(
X, y, test_size=0.5, random_state=0
)
# Create classifier
classifier = OneVsRestClassifier(
make_pipeline(StandardScaler(), LinearSVC(random_state=0))
)
# Train the model
classifier.fit(X_train, y_train)
# Make predictions
y_pred = classifier.predict(X_test)
y_pred_ = np.argmax(classifier.decision_function(X_test), axis=1)
print(classification_report(y_test, y_pred))
# precision recall f1-score support
# 0 1.00 1.00 1.00 21
# 1 0.85 0.73 0.79 30
# 2 0.71 0.83 0.77 24
# accuracy 0.84 75
# macro avg 0.85 0.86 0.85 75
# weighted avg 0.85 0.84 0.84 75
print(classification_report(y_test, y_pred_))
# precision recall f1-score support
# 0 1.00 1.00 1.00 21
# 1 0.85 0.73 0.79 30
# 2 0.71 0.83 0.77 24
# accuracy 0.84 75
# macro avg 0.85 0.86 0.85 75
# weighted avg 0.85 0.84 0.84 75
Answered By - Flavia Giammarino
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