How can i know probability of class predicted by predict() function in Support Vector Machine?

Question

How can i know sample's probability that it belongs to a class predicted by predict() function of Scikit-Learn in Support Vector Machine?

>>>print clf.predict([fv])
[5]

There is any function?

Answer 1

Definitely read this section of the docs as there's some subtleties involved. See also Scikit-learn predict_proba gives wrong answers

Basically, if you have a multi-class problem with plenty of data predict_proba as suggested earlier works well. Otherwise, you may have to make do with an ordering that doesn't yield probability scores from decision_function.

Here's a nice motif for using predict_proba to get a dictionary or list of class vs probability:

model = svm.SVC(probability=True)
model.fit(X, Y)
results = model.predict_proba(test_data)[0]

# gets a dictionary of {'class_name': probability}
prob_per_class_dictionary = dict(zip(model.classes_, results))

# gets a list of ['most_probable_class', 'second_most_probable_class', ..., 'least_class']
results_ordered_by_probability = map(lambda x: x[0], sorted(zip(model.classes_, results), key=lambda x: x[1], reverse=True))

Answer 2

Use clf.predict_proba([fv]) to obtain a list with predicted probabilities per class. However, this function is not available for all classifiers.

Regarding your comment, consider the following:

>> prob = [ 0.01357713, 0.00662571, 0.00782155, 0.3841413, 0.07487401, 0.09861277, 0.00644468, 0.40790285]
>> sum(prob)
1.0

The probabilities sum to 1.0, so multiply by 100 to get percentage.

Answer 3

For clearer answers, I post again the information from scikit-learn for svm.

Needless to say, the cross-validation involved in Platt scaling is an expensive operation for large datasets. In addition, the probability estimates may be inconsistent with the scores, in the sense that the “argmax” of the scores may not be the argmax of the probabilities. (E.g., in binary classification, a sample may be labeled by predict as belonging to a class that has probability <½ according to predict_proba.) Platt’s method is also known to have theoretical issues. If confidence scores are required, but these do not have to be probabilities, then it is advisable to set probability=False and use decision_function instead of predict_proba.

For other classifiers such as Random Forest, AdaBoost, Gradient Boosting, it should be okay to use predict function in scikit-learn.

Answer 4

When creating SVC class to compute the probability estimates by setting probability=True:

http://scikit-learn.org/stable/modules/generated/sklearn.svm.SVC.html

Then call fit as usual and then predict_proba([fv]).