SMOTE Oversampling & Tutorial On How To Implement In Python And R

by | Feb 17, 2023 | Data Science, Machine Learning

How does the algorithm work? What are the disadvantages and alternatives? And how do we use it in machine learning?

How does SMOTE work?

SMOTE stands for Synthetic Minority Over-sampling Technique. It is a technique used in machine learning and data mining to address the problem of imbalanced datasets, where the number of instances in the minority class is much smaller than the number of cases in the majority class.

Traditional machine learning algorithms may not perform well in such scenarios as they tend to be biased towards the majority class. SMOTE is an over-sampling technique that generates synthetic samples for the minority class by creating new instances similar to the existing ones. This helps balance the class distribution and improves the machine learning algorithm’s performance.

The SMOTE algorithm works by selecting a minority class instance at random and finding its k nearest minority class neighbours. It then generates new samples by interpolating between the selected instance and its k nearest neighbours in feature space. A user-defined parameter controls the number of new cases created for each minority instance.

SMOTE - new samples by interpolating between the selected instance and its k nearest neighbours

New samples by interpolating between the selected instance and its k nearest neighbours

SMOTE effectively improves the performance of machine learning algorithms on imbalanced datasets. However, it is very important to note that it is not a panacea, and its effectiveness may depend on the specific characteristics of the dataset and the machine learning algorithm being used.

The algorithm explained

Here is a step-by-step overview of the SMOT algorithm:

  1. For each minority class instance in the dataset, find its k nearest neighbours (k is a user-defined parameter). Any distance metric, like Euclidean or cosine distance, can be used to figure out how far apart two instances are.
  2. Select one of the k nearest neighbours at random, and generate a synthetic instance by interpolating between the selected sample and the original minority class instance. The interpolation is done by choosing random values for each feature, such that the values lie between the importance of the two cases in feature space. For example, if the selected neighbour has a feature value of 0.8 and the original instance has a feature value of 0.5, the synthetic sample might have a feature value of 0.6.
  3. Repeat step 2 for each minority class instance.
  4. Add the made-up examples to the original dataset to make a new dataset with the same number of each class.
  5. Train an algorithm for machine learning on the new data set and see how well it works on a test set.

SMOTE is a popular way to oversample, but over time, different versions of the algorithm, like Borderline-SMOTE and ADASYN, have been made to fix some of the problems with the original.

What are the disadvantages of SMOTE?

While the SMOTE (Synthetic Minority Over-sampling Technique) algorithm is a powerful technique for addressing the problem of imbalanced datasets in machine learning, it also has some limitations and potential drawbacks. Here are some of the disadvantages of SMOTE:

  1. Oversampling of noise: SMOTE can generate synthetic samples by interpolating between noisy examples, which can lead to over-representation of noise in the resampled dataset. This can negatively affect the performance of machine learning models trained on the resampled dataset.
  2. Dependence on the k parameter: The performance of SMOTE can depend heavily on the choice of the k parameter, which determines the number of nearest neighbors to use for generating synthetic samples. If the value of k is too low, the synthetic samples may not capture the full diversity of the minority class, while if it is too high, the synthetic samples may not be sufficiently diverse.
  3. Imbalance after resampling: While SMOTE can increase the number of minority class samples, it can also decrease the number of majority class samples, leading to a different type of imbalance in the resampled dataset. This can be addressed by using methods like random undersampling, but it is still a potential issue.
  4. Difficulty with high-dimensional data: SMOTE can be less effective in high-dimensional feature spaces, where the density of the minority class can be more difficult to estimate accurately.
  5. Inability to capture complex patterns: SMOTE generates synthetic samples by interpolating between neighboring examples, which may not capture the more complex patterns in the minority class.

Despite these limitations, SMOTE remains a useful tool for addressing the problem of imbalanced datasets in machine learning, and is often used in combination with other techniques to achieve better results. It is important to carefully evaluate the performance of machine learning models trained on resampled datasets, and to compare the performance of different resampling techniques to find the most effective one for a given problem.

What alternatives are there?

There are several alternatives to the SMOTE (Synthetic Minority Over-sampling Technique) algorithm for addressing the problem of imbalanced datasets in machine learning. Here are some of the commonly used techniques:

  1. Random undersampling: This technique removes samples from the majority class to balance the dataset. The downside of this approach is that it may lead to the loss of important information if the removed samples contain helpful information.
  2. Adaptive synthetic sampling (ADASYN): ADASYN is an improvement on SMOTE that changes how the minority class is spread out by making synthetic samples based on how hard it is for them to learn.
  3. Tomek links: With this method, you find and get rid of pairs of samples that are next to each other but are from different classes. This approach helps reduce the noise in the majority class.
  4. Edited nearest neighbour (ENN): This approach involves removing examples from the majority class misclassified by their nearest neighbours. This method can eliminate noise in the majority class and focus more on actual samples from the minority class.
  5. SMOTE-variants: Several algorithm variants, such as Borderline-SMOTE, Safe-Level-SMOTE, and ADOMS (Adaptive synthetic over-sampling using minority search).
  6. Cost-sensitive learning: Cost-sensitive learning is a technique that involves assigning different costs to different classes during training. This method works well when misclassifying the minority group is more expensive than misclassifying the majority group.

The choice of the most appropriate technique will depend on the dataset’s characteristics and the analysis’s goals. Before choosing the best method, you should always try a few different ones and compare how well they work.

Using SMOTE in machine learning

Synthetic Minority Over-sampling Technique is a technique used in machine learning to address the problem of imbalanced datasets. In imbalanced datasets, one class may have a much smaller number of samples than the other, resulting in a biased or inaccurate machine-learning model. SMOTE is an oversampling technique that generates synthetic samples for the minority class by creating new instances similar to the existing ones. This helps balance the class distribution and improves the machine learning algorithm’s performance.

The use of SMOTE in machine learning involves the following steps:

  1. Load and preprocess the imbalanced dataset, splitting it into training and testing sets.
  2. Use the SMOTE algorithm on the training set to make fake samples from the minority classes. This creates a new training set that is more balanced.
  3. Train a machine learning model on the balanced training set.
  4. Evaluate the performance of the trained model on the testing set.

SMOTE is a popular technique for addressing the problem of imbalanced datasets, and it effectively improves the performance of machine learning models. However, it is essential to note that it may not always be the best approach for addressing the problem of imbalanced datasets. Other techniques, such as undersampling or combining both, may also be effective. The best practice to use may depend on the specifics of the dataset and the machine learning algorithm being used.

How to use SMOTE in NLP

The SMOTE algorithm can also be applied to natural language processing (NLP) tasks that involve imbalanced datasets. Here are some ways in which SMOTE can be used in NLP:

  1. Text classification: SMOTE can be used to balance the number of positive and negative examples in a text classification task, such as sentiment analysis or spam detection. This can improve the performance of the classification model, especially when the number of positive examples is much smaller than the number of negative examples.
  2. Named entity recognition (NER): In NER tasks, the number of examples for some named entities can be much smaller than others, leading to an imbalanced dataset. SMOTE can be used to increase the number of examples for the minority named entities, which can improve the performance of the NER model.
  3. Question-answering: SMOTE can balance the number of examples for different types of questions in a question-answering task. For example, if there are few examples of questions related to a specific topic, SMOTE can be used to generate additional examples of that type of question.
  4. Text generation: SMOTE can balance the number of examples of different text types, such as positive and negative reviews, in a text generation task. This can improve the diversity and quality of the generated text.

When applying SMOTE to NLP tasks, it is important to consider the limitations and potential drawbacks of the algorithm, such as the oversampling of noise and the dependence on the k parameter. It may also be necessary to preprocess the text data before applying SMOTE, for example by using tokenization and feature extraction techniques.

How to use SMOTE in Python with imblearn and sklearn

The SMOTE algorithm can be used in Python with the help of the imblearn library, which has an implementation of the SMOTE algorithm.

Here’s an example of how to use it in Python:

from imblearn.over_sampling import SMOTE
from sklearn.datasets import make_classification
from sklearn.model_selection import train_test_split
from sklearn.linear_model import LogisticRegression
from sklearn.metrics import classification_report

# Generate an imbalanced classification dataset
X, y = make_classification(n_samples=10000, weights=[0.95], flip_y=0, random_state=1)

# Split the dataset into training and testing sets
X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.2, random_state=1)

# Apply SMOTE to the training set
smote = SMOTE()
X_train_resampled, y_train_resampled = smote.fit_resample(X_train, y_train)

# Train a logistic regression model on the resampled training set
model = LogisticRegression()
model.fit(X_train_resampled, y_train_resampled)

# Evaluate the performance of the trained model on the testing set
y_pred = model.predict(X_test)
print(classification_report(y_test, y_pred))

In this example, we first generate an imbalanced classification dataset using the make_classification function from scikit-learn. We then split the dataset into training and testing sets.

Next, we apply SMOTE to the training set using the SMOTE class from the imblearn.over_sampling module, and resample the training set to obtain a balanced dataset.

Finally, we train a logistic regression model on the resampled training set, and evaluate its performance on the testing set using the classification_report function from scikit-learn’s metrics module.

How to use SMOTE in R

In R, you can use the SMOTE function from the DMwR package to apply the SMOTE algorithm to address the problem of imbalanced datasets. Here’s an example of how to use it in R with the DMwR package:

library(DMwR)
library(caret)

# Load an imbalanced dataset
data(iris)
iris$Species <- ifelse(iris$Species == "setosa", "setosa", "versicolor")

# Split the dataset into training and testing sets
trainIndex <- createDataPartition(iris$Species, p = .8, list = FALSE)
training <- iris[trainIndex, ]
testing <- iris[-trainIndex, ]

# Apply SMOTE to the training set
training_balanced <- SMOTE(Species ~ ., training, k = 5, perc.over = 100, perc.under = 200)

# Train a decision tree model on the resampled training set
model <- train(Species ~ ., training_balanced, method = "rpart")

# Evaluate the performance of the trained model on the testing set
predictions <- predict(model, testing)
confusionMatrix(predictions, testing$Species)

In this example, we first load an imbalanced dataset (the iris dataset) and convert it to a binary classification problem by labelling the “setosa” class as positive and the “versicolor” class as negative.

We then split the dataset into training and testing sets using the createDataPartition function from the caret package. Next, we apply SMOTE to the training set using the SMOTE function from the DMwR package, with the k parameter set to 5 and the perc.over and perc.under parameters set to 100 and 200, respectively. Finally, we train a decision tree model on the resampled training set using the train function from the caret package, and evaluate its performance on the testing set using the confusionMatrix function from the caret package.

Note that you need to install the DMwR and caret packages separately before using them in your code. You can install them using the install.packages function in R. To install the DMwR package, for example, use install.packages(“DMwR”) in R.

Conclusion

The SMOTE (Synthetic Minority Over-sampling Technique) algorithm is a powerful technique for addressing the problem of imbalanced datasets in machine learning. By making fake samples of the minority class, SMOTE can improve how well machine learning models work on datasets that aren’t balanced.

In addition to SMOTE, several other techniques can be used to address the problem of imbalanced datasets, including random undersampling, adaptive synthetic sampling (ADASYN), Tomek links, edited nearest neighbour (ENN), SMOTE-variants, and cost-sensitive learning. The dataset’s characteristics and the analysis’s goals will determine which technique is best.

When applying SMOTE in R, you can use the function from the DMwR package to resample the minority class and balance the dataset. Then, you can use the resampled dataset to train and test different machine learning models in R.

Overall, handling imbalanced datasets is an essential and challenging task in machine learning, and using techniques like SMOTE can help improve the performance of machine learning models on these datasets.

About the Author

Neri Van Otten

Neri Van Otten

Neri Van Otten is the founder of Spot Intelligence, a machine learning engineer with over 12 years of experience specialising in Natural Language Processing (NLP) and deep learning innovation. Dedicated to making your projects succeed.

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