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Stratified K Fold Cross Validation
• Difficulty Level : Easy
• Last Updated : 05 Sep, 2020

In machine learning, When we want to train our ML model we split our entire dataset into training_set and test_set using `train_test_split()` class present in `sklearn`.Then we train our model on training_set and test our model on test_set. The problems that we are going to face in this method are:

Whenever we change the random_state parameter present in `train_test_split()`, We get different accuracy for different random_state and hence we can’t exactly point out the accuracy for our model.
The `train_test_split()` splits the dataset into `training_test` and `test_set` by random sampling.But stratified sampling is performed.

What is random sampling and Stratified sampling ?
Suppose you want to take a survey and decided to call 1000 people from a particular state, If you pick either 1000 male completely or 1000 female completely or 900 female and 100 male (randomly) to ask their opinion on a particular product.Then based on these 1000 opinion you can’t decide the opinion of that entire state on your product.This is random sampling.

But in Stratified Sampling, Let the population for that state be 51.3% male and 48.7% female, Then for choosing 1000 people from that state if you pick 531 male ( 51.3% of 1000 ) and 487 female ( 48.7% for 1000 ) i.e 531 male + 487 female (Total=1000 people) to ask their opinion. Then these groups of people represent the entire state. This is called as Stratified Sampling.

Why random sampling is not prefered in machine learning ?
Let’s consider a binary-class classification problem. Let our dataset consists of 100 samples out of which 80 are negative class { 0 } and 20 are positive class { 1 }.

Random sampling:
If we do random sampling to split the dataset into training_set and test_set in 8:2 ratio respectively.Then we might get all negative class {0} in training_set i.e 80 samples in training_test and all 20 positive class {1} in test_set.Now if we train our model on training_set and test our model on test_set, Then obviously we will get a bad accuracy score.

Stratified Sampling:
In stratified sampling, The training_set consists of 64 negative class{0} ( 80% 0f 80 ) and 16 positive class {1} ( 80% of 20 ) i.e. 64{0}+16{1}=80 samples in training_set which represents the original dataset in equal proportion and similarly test_set consists of 16 negative class {0} ( 20% of 80 ) and 4 positive class{1} ( 20% of 20 ) i.e. 16{0}+4{1}=20 samples in test_set which also represents the entire dataset in equal proportion.This type of train-test-split results in good accuracy.

What is the solution for mentioned problems?
The solution for the first problem where we were able to get different accuracy score for different random_state parameter value is to use K-Fold Cross-Validation. But K-Fold Cross Validation also suffer from second problem i.e. random sampling.

The solution for both first and second problem is to use Stratified K-Fold Cross-Validation.

What is Stratified K-Fold Cross Validation?
Stratified k-fold cross-validation is same as just k-fold cross-validation, But in Stratified k-fold cross-validation, it does stratified sampling instead of random sampling.

Code: Python code implementation of Stratified K-Fold Cross Validation

 `# This code may not be run on GFG IDE  ``# as required packages are not found.  ``    ` `# STRATIFIES K-FOLD CROSS VALIDATION { 10-fold }``  ` `# Import Required Modules.``from` `statistics ``import` `mean, stdev``from` `sklearn ``import` `preprocessing``from` `sklearn.model_selection ``import` `StratifiedKFold``from` `sklearn ``import` `linear_model``from` `sklearn ``import` `datasets``  ` `# FEATCHING FEATURES AND TARGET VARIABLES IN ARRAY FORMAT.``cancer ``=` `datasets.load_breast_cancer()``# Input_x_Features.``x ``=` `cancer.data                         `` ` `# Input_ y_Target_Variable.``y ``=` `cancer.target                       ``   ` `  ` `# Feature Scaling for input features.``scaler ``=` `preprocessing.MinMaxScaler()``x_scaled ``=` `scaler.fit_transform(x)``  ` `# Create  classifier object.``lr ``=` `linear_model.LogisticRegression()``  ` `# Create StratifiedKFold object.``skf ``=` `StratifiedKFold(n_splits``=``10``, shuffle``=``True``, random_state``=``1``)``lst_accu_stratified ``=` `[]``  ` `for` `train_index, test_index ``in` `skf.split(x, y):``    ``x_train_fold, x_test_fold ``=` `x_scaled[train_index], x_scaled[test_index]``    ``y_train_fold, y_test_fold ``=` `y[train_index], y[test_index]``    ``lr.fit(x_train_fold, y_train_fold)``    ``lst_accu_stratified.append(lr.score(x_test_fold, y_test_fold))``  ` `# Print the output.``print``(``'List of possible accuracy:'``, lst_accu_stratified)``print``(``'\nMaximum Accuracy That can be obtained from this model is:'``,``      ``max``(lst_accu_stratified)``*``100``, ``'%'``)``print``(``'\nMinimum Accuracy:'``,``      ``min``(lst_accu_stratified)``*``100``, ``'%'``)``print``(``'\nOverall Accuracy:'``,``      ``mean(lst_accu_stratified)``*``100``, ``'%'``)``print``(``'\nStandard Deviation is:'``, stdev(lst_accu_stratified))`

Output:

```List of possible accuracy: [0.9298245614035088, 0.9649122807017544, 0.9824561403508771, 1.0, 0.9649122807017544, 0.9649122807017544, 0.9824561403508771, 0.9473684210526315, 0.9473684210526315, 0.9821428571428571]

Maximum Accuracy That can be obtained from this model is: 100.0 %

Minimum Accuracy That can be obtained from this model is: 92.98245614035088 %

The overall Accuracy of this model is: 96.66353383458647 %

The Standard Deviation is: 0.02097789213195869```

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