If k-folds cross-validation gives k models, how can we decide which to use? I clearly have a major misunderstanding of k-folds cross-validation. Suppose that you have some training data, and you use 5-folds cross-validation to train a model with it. Unless I am very much mistaken, you now have five models. So how are you going to decide which model to use in practice? You may have some test data, but picking a model based on that data is rightly considered cheating (the test data should always be truly independent).
 A: Cross validation (say k-fold) serves the following purposes in general:


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*Tuning your hyper-parameters. In this one, you use the training data, apply cv and decide on the best hyper-parameters based on average validation set performance (using k different models and k different validation sets) and pick the best one. Once chosen, train on the whole training data with chosen hyper-parameters and evaluate on the test data. So, there is one model for the testing.

*Estimating the test performance. Typically done when the data is scarce, and you don't want to separate a single test set with small number of samples. No hyper-parameter search is done here (unless inner CV is applied). Here, you choose a model apply k-fold cv and get a k validation scores and get the average (or predict all the dataset using k-fold and then calculate the cumulative score afterwards, mainly done when scores corresponding to a single validation fold are not preferred, e.g. correlation score when LOOCV is used). This result is a promise/estimate of the success on a test data that you do not have access right now. In this one, you have k models, but it doesn't matter since you don't have a separate test data.

