How to Handle an Unbalanced Dataset by Downsampling?



In this article, we will go over how we can handle unbalanced datasets through downsampling. We will see how to downsample with Python, why it is important, and when to downsample.


Why is downsampling important?

Downsampling an imbalanced dataset is essential because many machine learning algorithms assume that the data is balanced and will perform poorly if this assumption is not met. For example, when training a classification model on an imbalanced dataset, the model may learn to predict the majority class most of the time, leading to poor performance in the minority class.


Downsampling can address this problem by reducing the size of the majority class and creating a new, balanced dataset. It allows the machine learning algorithm to learn from a more representative sample of the data and can improve its performance on the minority class.


In addition to improving model performance, downsampling can also make the training process more efficient by reducing the size of the dataset and allowing the model to train faster. This process can be beneficial when working with large datasets that may be difficult to manage. Overall, downsampling is a handy technique for handling imbalanced data and improving the performance of machine learning algorithms.


How to handle an unbalanced dataset by downsampling using Python?

To handle an unbalanced dataset using downsampling, you first need to identify the class with the most observations (the majority class), then randomly select a subset of observations from that class so that it is the same size as the minority class. The output of this process will create a new, balanced dataset with an equal number of observations from each category.


To implement this in practice, you would first need to calculate the size of the minority class and then use a random number generator to select a subset of observations from the majority class with the same size. For example, in Python, you could use the pandas library to choose a random subset of observations from a dataframe like this:

# calculate the size of the minority class
minority_size = df[df['label'] == 'minority'].shape[0]

# randomly select a subset of observations from the majority class
majority_subset = df[df['label'] == 'majority'].sample(n=minority_size, random_state=123)

# combine the minority and majority subsets to create a new, balanced dataset
balanced_df = pd.concat([majority_subset, df[df['label'] == 'minority']])


After downsampling, it is vital to check that the new dataset is balanced and contains an equal number of observations from each class. You can do this by using the value_counts() method on the target variable to see the frequency of each class.


The above code should return a series with two values, showing the number of observations from each class in the new, downsampled dataset.


When should you downsample an imbalanced dataset?

You should downsample an imbalanced dataset when you are training a machine learning algorithm, and the imbalanced distribution of the data is causing problems. For example, if you are training a classification model and the majority class dominates the training data, the model may have difficulty predicting the minority class.


Downsampling can help to improve the performance of the machine learning algorithm on the minority class and lead to more accurate predictions.


There are a few key considerations to keep in mind when deciding whether to downsample an imbalanced dataset. First, you should only downsample if the majority class is significantly larger than the minority class, as downsampling will not be effective if the difference in class sizes is slight. Second, you should avoid downsampling if the minority class is tiny, as this can reduce the data available for training the model and may lead to poor performance.


Overall, downsampling is a valuable technique for addressing imbalanced data in machine learning, but it should be used carefully to ensure that it does not negatively impact the model’s performance.


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