How is scikit-learn cross_val_predict accuracy sco

问题:

Does the cross_val_predict (see doc, v0.18) with k-fold method as shown in the code below calculate accuracy for each fold and average them finally or not?

cv = KFold(len(labels), n_folds=20)
clf = SVC()
ypred = cross_val_predict(clf, td, labels, cv=cv)
accuracy = accuracy_score(labels, ypred)
print accuracy

回答1:

No, it does not!

According to cross validation doc page, cross_val_predict does not return any scores but only the labels based on a certain strategy which is described here:

The function cross_val_predict has a similar interface to cross_val_score, but returns, for each element in the input, the prediction that was obtained for that element when it was in the test set. Only cross-validation strategies that assign all elements to a test set exactly once can be used (otherwise, an exception is raised).

And therefore by calling accuracy_score(labels, ypred) you are just calculating accuracy scores of labels predicted by aforementioned particular strategy compared to the true labels. This again is specified in the same documentation page:

These prediction can then be used to evaluate the classifier:

predicted = cross_val_predict(clf, iris.data, iris.target, cv=10) 
metrics.accuracy_score(iris.target, predicted)

Note that the result of this computation may be slightly different from those obtained using cross_val_score as the elements are grouped in different ways.

If you need accuracy scores of different folds you should try:

>>> scores = cross_val_score(clf, X, y, cv=cv)
>>> scores                                              
array([ 0.96...,  1.  ...,  0.96...,  0.96...,  1.        ])

and then for the mean accuracy of all folds use scores.mean():

>>> print("Accuracy: %0.2f (+/- %0.2f)" % (scores.mean(), scores.std() * 2))
Accuracy: 0.98 (+/- 0.03)

Edit:

For calculating Cohen Kappa coefficient and confusion matrix I assumed you mean kappa coefficient and confusion matrix between true labels and each fold's predicted labels:

from sklearn.model_selection import KFold
from sklearn.svm.classes import SVC
from sklearn.metrics.classification import cohen_kappa_score
from sklearn.metrics import confusion_matrix

cv = KFold(len(labels), n_folds=20)
clf = SVC()
for train_index, test_index in cv.split(X):
    clf.fit(X[train_index], labels[train_index])
    ypred = clf.predict(X[test_index])
    kappa_score = cohen_kappa_score(labels[test_index], ypred)
    confusion_matrix = confusion_matrix(labels[test_index], ypred)

Edit 2:

What does cross_val_predict return?

KFold splits the data to k parts and then for i=1..k iterations does this: takes all parts except i'th part as the training data, fits the model with them and then predicts labels for i'th part (test data). In each iteration, label of i'th part of data gets predicted. In the end cross_val_predict merges all partially predicted labels and returns them as a whole.

This code shows this process step by step:

X = np.array([[0], [1], [2], [3], [4], [5]])
labels = np.array(['a', 'a', 'a', 'b', 'b', 'b'])

cv = KFold(len(labels), n_folds=3)
clf = SVC()
ypred_all = np.chararray((labels.shape))
i = 1
for train_index, test_index in cv.split(X):
    print("iteration", i, ":")
    print("train indices:", train_index)
    print("train data:", X[train_index])
    print("test indices:", test_index)
    print("test data:", X[test_index])
    clf.fit(X[train_index], labels[train_index])
    ypred = clf.predict(X[test_index])
    print("predicted labels for data of indices", test_index, "are:", ypred)
    ypred_all[test_index] = ypred
    print("merged predicted labels:", ypred_all)
    i = i+1
    print("=====================================")
y_cross_val_predict = cross_val_predict(clf, X, labels, cv=cv)
print("predicted labels by cross_val_predict:", y_cross_val_predict)

The result is:

iteration 1 :
train indices: [2 3 4 5]
train data: [[2] [3] [4] [5]]
test indices: [0 1]
test data: [[0] [1]]
predicted labels for data of indices [0 1] are: ['b' 'b']
merged predicted labels: ['b' 'b' '' '' '' '']
=====================================
iteration 2 :
train indices: [0 1 4 5]
train data: [[0] [1] [4] [5]]
test indices: [2 3]
test data: [[2] [3]]
predicted labels for data of indices [2 3] are: ['a' 'b']
merged predicted labels: ['b' 'b' 'a' 'b' '' '']
=====================================
iteration 3 :
train indices: [0 1 2 3]
train data: [[0] [1] [2] [3]]
test indices: [4 5]
test data: [[4] [5]]
predicted labels for data of indices [4 5] are: ['a' 'a']
merged predicted labels: ['b' 'b' 'a' 'b' 'a' 'a']
=====================================
predicted labels by cross_val_predict: ['b' 'b' 'a' 'b' 'a' 'a']

回答2:

As you can see from the code of cross_val_predict on github, the function computes for each fold the predictions and concatenates them. The predictions are made based on model learned from other folds.

Here is a combination of your code and the example provided in the code

from sklearn import datasets, linear_model
from sklearn.model_selection import cross_val_predict, KFold
from sklearn.metrics import accuracy_score

diabetes = datasets.load_diabetes()
X = diabetes.data[:400]
y = diabetes.target[:400]
cv = KFold(n_splits=20)
lasso = linear_model.Lasso()
y_pred = cross_val_predict(lasso, X, y, cv=cv)
accuracy = accuracy_score(y_pred.astype(int), y.astype(int))

print(accuracy)
# >>> 0.0075

Finally, to answer your question: "No, the accuracy is not averaged for each fold"

回答3:

I would like to add an option for a quick and easy answer, above what the previous developers contributed.

If you take micro average of F1 you will essentially be getting the accuracy rate. So for example that would be:

from sklearn.model_selection import cross_val_score, cross_val_predict
from sklearn.metrics import precision_recall_fscore_support as score    

y_pred = cross_val_predict(lm,df,y,cv=5)
precision, recall, fscore, support = score(y, y_pred, average='micro') 
print(fscore)

This works mathematically, since the micro average gives you the weighted average of the confusion matrix.

Good luck.

回答4:

As it is written in the documenattion sklearn.model_selection.cross_val_predict :

It is not appropriate to pass these predictions into an evaluation metric. Use cross_validate to measure generalization error.