I have read on the several answers here and on the Internet that cross-validation helps to indicate that if the model will generalize well or not and about overfitting.

But I am confused that which two accuracies/errors amoung test/training/validation should I compare to be able to see if the model is overfitting or not?

For example:

I divide my data for 70% training and 30% test.

When I get to run 10 fold cross-validation, I get 10 accuracies that I can take the average/mean of. should I call this mean as validation accuracy?

Afterward, I test the model on 30% test data and get Test Accuracy.

In this case, what will be training accuracy? And which two accuracies should I compare to see if the model is overfitting or not?


2 Answers 2


Which two accuracies I compare to see if the model is overfitting or not?

You should compare the training and test accuracies to identify over-fitting. A training accuracy that is subjectively far higher than test accuracy indicates over-fitting.

Here, "accuracy" is used in a broad sense, it can be replaced with F1, AUC, error (increase becomes decrease, higher becomes lower), etc.

I suggest "Bias and Variance" and "Learning curves" parts of "Machine Learning Yearning - Andrew Ng". It presents plots and interpretations for all the cases with a clear narration.

When I get to run 10 fold cross-validation, I get 10 accuracies that I can take the average/mean of. should I call this mean as validation accuracy?

No. It is a [estimate of] test accuracy.
The difference between validation and test sets (and their corresponding accuracies) is that validation set is used to build/select a better model, meaning it affects the final model. However, since 10-fold CV always tests an already-built model on its 10% held-out, and it is not used here to select between models, its 10% held-out is a test set not a validation set.

Afterward, I test the model on 30% test data and get Test Accuracy.

If you don't use the K-fold to select between multiple models, this part is not needed, run K-fold on 100% of data to get the test accuracy. Otherwise, you should keep this test set, since the result of K-fold would be a validation accuracy.

In this case, what will be training accuracy?

From each of 10 folds you can get a test accuracy on 10% of data, and a training accuracy on 90% of data. In python, method cross_val_score only calculates the test accuracies. Here is how to calculate both:

from  sklearn import model_selection
from sklearn import datasets
from sklearn import svm

iris = datasets.load_iris()
clf = svm.SVC(kernel='linear', C=1)
scores = model_selection.cross_validate(clf, iris.data, iris.target, cv=5, return_train_score=True)
print('Train scores:')
print('Test scores:')

Set return_estimator = True to get the trained models too.

More on validation set

Validation set shows up in two general cases: (1) building a model, and (2) selecting between multiple models,

  1. Two examples for building a model: we (a) stop training a neural network, or (b) stop pruning a decision tree when accuracy of model on validation set starts to decrease. Then, we test the final model on a held-out set, to get the test accuracy.

  2. Two examples for selecting between multiple models:

    a. We do K-fold CV on one neural network with 3 layers, and one with 5 layers (to get K models for each), then we select the NN with the highest validation accuracy averaged over K models; suppose the 5 layer NN. Finally, we train the 5 layer NN on a 80% train, 20% validation split of combined K folds, and then test it on a held out set to get the test accuracy.

    b. We apply two already-built SVM and decision tree models on a validation set, then we select the one with the highest validation accuracy. Finally, we test the selected model on a held-out set to get the test accuracy.

  • 4
    $\begingroup$ I think I disagree with "30% test set not needed." If you are using CV to select a better model, then you are exposing the test folds (which I would call a validation set in this case) and risk overfitting there. The final test set should remain untouched (by both you and your algorithms) until the end, to estimate the final model performance (if that's something you need). But yes, while model-building, the (averaged) training fold score vs. the (averaged) validation fold score is what you're looking at for overfitting indication. $\endgroup$
    – Ben Reiniger
    Mar 13, 2019 at 20:21
  • 1
    $\begingroup$ @BenReiniger You are right I should clear this case. $\endgroup$
    – Esmailian
    Mar 13, 2019 at 20:23
  • $\begingroup$ @Esmailian train_score is also an average of 10 scores? Also, to do a similar kind of thing with GridSearchCV(in case hyper paramter tuning and cross-validation are required in one step) can we use return_train_score=true? is it same? $\endgroup$
    – A.B
    Mar 13, 2019 at 22:22
  • $\begingroup$ @A.B It is an array, needs to be averaged. return_train_score=true or =false only changes the returned report, underlying result is the same. $\endgroup$
    – Esmailian
    Mar 13, 2019 at 22:27
  • 1
    $\begingroup$ Okay thanks, I am accepting the answer as "which accuracy is to be used" makes sense. But is it possible for you to elaborate more on "validation set is used to build/select a better model (e.g. avoid over-fitting) vs in your case, 10-fold CV tests an already-built model" for me and future readers? $\endgroup$
    – A.B
    Mar 13, 2019 at 22:32

Cross validation splits your data into K folds. Each fold contains a set of training data and test data. You are correct that you get K different error rates that you then take the mean of. These error rates come from the test set of each of your K folds. If you want to get the training error rate, you would calculate the error rate on the training part of each of these K folds and then take the average.


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