# Why k-fold cross validation (CV) overfits? Or why discrepancy occurs between CV and test set?

Recently, I was working on a project and found my cross-validation error rate very low, but the testing set error rate very high. This might indicate that my model is overfitting. Why does my cross-validation not overfit while my test set overfits?

More specifically, I have about 2 million observations with 100 variables (n>>p). I randomly split the data set into 80/20 train and test. Then, I fit a model (i.e. XGboost) using a 5-fold cross validation on the training set and the estimated error rate is pretty low. Then, I used the same parameter setting and used the entire training set to fit the model. Surprisingly, when I used the test set to evaluate the performance of the model, the error rate is significantly higher than the CV error rate. Why?

Edit:

The error rate is actually multinomial logloss. I achieved a CV error rate of 1.320044 (+/- 0.002126) and a testing error rate of 1.437881. They might seem close by staring at these two numbers, but actually, they are not. I don't know how to justify this, but I am sure that they are different within the scale of performance of this project, which is from ~1.55 to ~1.30.

The way of 5-fold cross validation is like following,

1. divide the train set into 5 sets.
2. iteratively fit a model on 4 sets and test the performance on the rest set.
3. average the performance of all the five iterations.

I mean, if my parameter settings make model overfit, then I should see it at this cross-validation procedure, right? But I don't see it until I use the test set. Under what circumstances on the earth this could happen?

Thanks!

The only reason I could think of that why CV error rate diffs from test set error rate is

Cross-Validation will not perform well to outside data if the data you do have is not representative of the data you'll be trying to predict! -- here

But I randomly 8/2 split the 2-million-sample data set and I believe that the train set and test set should have a same distribution of variables.

(Pardon me that I post the same question here also.)

Edit:

I got one interesting commend from @darXider in cross validated. He says,

Sometimes when feature engineering, you have to be careful to avoid any data leak between training and test sets. For example, if you do a PCA on your original, untouched data, use PC1 and PC2 as "new" features, and then split your dataset into train and test, you are leaking information from training set into test set. That will boost your score up. You mentioned that after some feature engineering, your CV score and test score started to disagree. That could suggest some sort of information leak between training set and test set.

I wonder what is exactly the "data leakage" and why feature engineering BEFORE splitting can still cause "data leakage"??

• The SD of +/- 0.002 seems pretty good for 5-fold CV given your mean of 1.32 - could you give the individual fold scores? Mar 4, 2017 at 8:51