# For imbalanced classification, should the validation dataset be balanced?

I am building a binary classification model for imbalanced data (e.g., 90% Pos class vs 10% Neg Class).

I already balanced my training dataset to reflect a a 50/50 class split, while my holdout (training dataset) was kept similar to the original data distribution (i.e., 90% vs 10%). My question is regarding the validation data used during the CV hyperparameter process. During each iteration fold should:

1) Both the training and test folds be balanced

or

2) The training fold should be kept balanced while the validation fold should be made imbalanced to reflect the original data distribution and holdout dataset.

I am currently using the 1st option to tune my model; however, is this approach valid given that the holdout and validation datasets have different distributions?

Both test and validation datasets should have the same distribution. In such a case, the performance metrics on the validation dataset are a good approximation of the performance metrics on the test dataset. However, the training dataset can be different. Also, it is fine and sometimes helpful to balance the training dataset. On the other hand, balancing the test dataset could lead to a bias estimation from the performance of the model because the test dataset should reflect the original data imbalance. As I mentioned at the beginning the test and validation datasets should have the same distribution. Since balancing the test dataset is not allowed, the validation dataset can not be validated too.

Additionally, I should mention that when you balance the test dataset, you will get a better performance in comparison to using an unbalanced dataset for testing. And of course, using a balanced test set does not make sense as explained above. So, the resulted performance is not reliable unless you use an unbalanced dataset with the same distribution of classes as the actual data.

• how can you use the pipeline in Sklearn to just balance the training folds but not the validation fold. Are you referring to using SMOTE or down-, up-sampling methods? – thereandhere1 Jun 16 at 1:52
• I mentioned k-fold cross-validation. In that case, we need to have an unbalanced validation fold; while the rest of the folds (k-1 folds) need to be balanced. I remove that line as it might lead to confusion. – nimar Jun 16 at 4:22
• Thank you for the clarification. I also created a follow up question (datascience.stackexchange.com/questions/76107/…) for additional clarification on the recommended training schema, I would appreciate your additional input. – thereandhere1 Jun 16 at 16:08

In my opinion the validation set should follow the original imbalanced distribution: the goal is ultimately to apply the model to the real distribution so the hyper-parameters should be chosen to maximize performance for this distribution.

But since I'm not completely sure I'd suggest trying both options, and adopt the one which gives the best performance on the test set.

• At one side you are right but another as u said above validation set should follow the imbalance distribution that I disagree because by doing that we can not get degree of generalization of model. What u say about it? – Gaurav Koradiya Jun 16 at 4:46
• @GauravKoradiya well I think there are arguments for the two options. in doubt I would try both, as I said. – Erwan Jun 16 at 12:04