# K-Fold cross validation and data leakage

I want to do K-Fold cross validation and also I want to do normalization or feature scaling for each fold. So let's say we have k folds. At each step we take one fold as validation set and the remaining k-1 folds as training set. Now I want to do feature scaling and data imputation on that training set and then apply the same transformation on that validation set. I want to do this for each step. I am trying to avoid data leakage as much as possible and at the same time rescale my validation sets to get better results.

How can I do this with a few lines of code?

Secondly, is it necessary to do this? Because I don't see many people do this for k-fold validation. I have seen many times, they do feature scaling and imputation on the entire dataset first and then do the k-fold cross validation. But doesn't this cause data leakage?

• In what language you want this? – Julio Jesus Dec 23 '20 at 23:40
• If you are using python, sklearn's pipelines are the answer – Julio Jesus Dec 23 '20 at 23:41

A reproducible example with no data leakage:

In there I'm scaling the data only with the train data on the k-fold stage

import numpy as np

from sklearn.preprocessing import StandardScaler
from sklearn.linear_model import LogisticRegression
from sklearn.model_selection import train_test_split, cross_val_score
from sklearn.pipeline import Pipeline
from sklearn.impute import SimpleImputer

X_train, X_test, y_train, y_test = train_test_split(X, y, test_size = .2, random_state = 42)

model = Pipeline([("imputing", SimpleImputer(strategy= "mean")),("scaling", StandardScaler()), ("modeling", LogisticRegression(random_state= 42, class_weight= "balanced"))]).fit(X_train, y_train)

cv_scores = cross_val_score(estimator= model, X = X_train, y = y_train, scoring= "accuracy")

print(f"Mean accuracy cv: {np.mean(cv_scores)}")

model.score(X_test, y_test)


Note that in this case it is easy to apply all the pipeline because all the feature are the same data type, but imagine you have categorical and continuous features, so you need to apply different preprocessing and imputing to each.

In that case a combination of ColumnTransformer and Pipeline would do the job.

• Thank you for the answer. Can you please tell me how do you use imputation in the pipeline as well? – user109341 Dec 24 '20 at 0:32
• Sure thing! I recommend you to check the documentation on pipelines, and said by Andreas Muller itself "If you are not using pipelines you are probably doing it wrong" – Julio Jesus Dec 24 '20 at 1:27
• You can do practically everything you can imagine inside a Pipeline, for reference check some of my answers in which for example I used Pipeline to generate a 3D plot using PCA (If you find it relevant please upvote this to help me continue to aid others) – Julio Jesus Dec 24 '20 at 1:33
• Great! Thank you @Julio – user109341 Dec 24 '20 at 2:26
• @MDan if the answer resolved your issue, kindly accept it - see What should I do when someone answers my question? – desertnaut Dec 24 '20 at 11:56