By Saeed Mirshekari

Jun 3, 2023


Machine learning algorithms play a pivotal role in extracting insights from vast amounts of data, allowing us to make informed decisions in various domains. Two crucial concepts that shape the performance of these algorithms are bias and variance. In this blog post, we will explore the concepts of bias and variance in machine learning, their trade-off, and provide examples from Kaggle datasets to illustrate their impact on model performance.

Understanding Bias:

Bias refers to the assumptions made by a model when attempting to represent the underlying relationships between features and the target variable. A model with high bias oversimplifies the underlying data patterns, leading to underfitting. This often results in the model being unable to capture the complexities present in the data, leading to poor performance.

Example: Titanic Dataset

Consider the famous Titanic dataset from Kaggle, which contains information about the passengers and their survival status. If we use a simple logistic regression model with only one feature, such as age, to predict survival, the model may have high bias. It assumes a linear relationship between age and survival, disregarding other crucial factors like gender, class, and the availability of lifeboats. Consequently, the model will be too simplistic and fail to capture the complexities, resulting in poor predictive performance.

Understanding Variance:

Variance, on the other hand, refers to the sensitivity of a model to fluctuations in the training data. A model with high variance overfits the training data, meaning it fits the noise or random variations present in the data rather than the underlying patterns. This leads to poor generalization and a lack of accuracy when applied to unseen data.

Example: House Prices Dataset

Let's consider the House Prices dataset, where the goal is to predict the prices of houses based on various features. If we train a complex model, such as a deep neural network, with a small training dataset, the model may exhibit high variance. Due to limited data, the model might memorize the training examples, including the noise, rather than learning the true underlying relationships. As a result, when presented with new, unseen data, the model will likely fail to generalize, leading to poor predictions.

Bias-Variance Trade-Off:

Achieving optimal performance in machine learning requires striking a balance between bias and variance. The bias-variance trade-off suggests that as we reduce bias, variance tends to increase, and vice versa. Finding the right level of complexity in the model is crucial to achieve the best trade-off.

Example: Iris Dataset

Consider the Iris dataset, which aims to classify iris flowers into three different species. If we use a decision tree algorithm with a shallow depth, the model will have high bias, leading to underfitting. It might not capture the subtle differences between the species, resulting in poor accuracy. On the other hand, if we increase the depth of the decision tree excessively, the model may have high variance, overfitting the training data. It will memorize the training samples, including the noise, leading to poor generalization on unseen instances.


Bias and variance are vital concepts in machine learning that impact the performance of models. While bias relates to the simplicity of the model's assumptions, variance captures its sensitivity to training data fluctuations. Achieving the right balance between these two factors is crucial for building robust and accurate models. By understanding the trade-off between bias and variance, we can choose appropriate algorithms, tune hyperparameters, and employ techniques like cross-validation to strike an optimal balance. Kaggle datasets serve as valuable resources to analyze and illustrate these concepts, providing real-world examples for exploring the intricacies of bias and variance in machine learning.

About O'Fallon Labs

In O'Fallon Labs we help recent graduates and professionals to get started and thrive in their Data Science careers via 1:1 mentoring and more.

Saeed Mirshekari, PhD

Saeed is currently a Director of Data Science in Mastercard and the Founder & Director of O'Fallon Labs LLC. He is a former research scholar at LIGO team (Physics Nobel Prize of 2017). Learn more about Saeed...

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