Striking the Balance: Overfitting and Underfitting in Machine Learning

Parth Sojitra
7 min readNov 21, 2023


In the realm of machine learning and data science, the ultimate goal is to create models that generalize well to unseen data. These models should not only fit the training data but also make accurate predictions on new, unseen data. However, achieving this balance between capturing underlying patterns and avoiding noise in data is a challenge. This challenge is encapsulated in two common adversaries: overfitting and underfitting.

Overfitting occurs when a model is overly complex, effectively fitting the noise in the data along with the underlying patterns. It’s like trying to memorize answers rather than truly understanding the questions. On the opposite end of the spectrum, underfitting happens when a model is overly simplistic and fails to capture the underlying patterns in the data. It’s akin to using a straight edge to measure the curvature of the Earth.

In this comprehensive exploration, we will delve into the world of overfitting and underfitting. We’ll uncover their root causes, consequences, and strategies to mitigate them. Whether you’re an aspiring data scientist or an experienced machine learning practitioner, understanding these adversaries is essential. They are the gatekeepers that determine whether your model merely works or excels, making all the difference in the realm of data-driven decision-making.

Understanding Model Complexity

The essence of overfitting and underfitting lies in the complexity of machine learning models. Most machine learning algorithms have the flexibility to adjust their complexity based on the data they are trained on. This adaptability is both a strength and a potential pitfall.

When a model is too complex, it can fit the training data almost perfectly, including the noise. This leads to overfitting. On the other hand, when a model is too simple, it may struggle to capture the underlying patterns in the data, resulting in underfitting. The key challenge is finding the right level of complexity that allows a model to generalize well to new data.

Overfitting: The Curse of Complexity

What is Overfitting?

Overfitting occurs when a model captures not only the underlying patterns in the data but also the noise or random fluctuations present in the training data. In essence, it becomes too complex, fitting the training data so closely that it loses its ability to make accurate predictions on new, unseen data.

Causes of Overfitting

Several factors can contribute to overfitting:

  1. Excessive Model Complexity: When a model has too many parameters or is too deep (in the case of neural networks), it can fit the training data extremely closely, capturing even the smallest variations.
  2. Too Many Features: Having a large number of features, especially when some of them are irrelevant or noisy, can lead to overfitting. The model may find patterns in the noise.
  3. Insufficient Data: With a limited amount of data, it becomes easier for a complex model to fit the noise. More data can often mitigate overfitting.

Indicators of Overfitting

How can you tell if your model is overfitting? There are several indicators:

  • High Training Accuracy, Low Testing Accuracy: If your model achieves close to 100% accuracy on the training data but performs poorly on a separate test dataset, it’s likely overfitting.
  • Excessive Model Complexity: If your model has a large number of parameters or a high degree of complexity, it’s more prone to overfitting.
  • Spiky or Wiggly Predictions: When you visualize the model’s predictions, you might notice that it exhibits spiky or wiggly behavior instead of a smooth curve.
  • Large Differences in Performance: If there is a significant gap between the model’s performance on the training data and the test data, it’s a sign of overfitting.

Consequences of Overfitting

Overfitting has several adverse consequences:

  1. Poor Generalization: Overfit models do not generalize well to new, unseen data. They are highly specialized for the training data and often fail to make accurate predictions on real-world data.
  2. Loss of Interpretability: Overly complex models can be challenging to interpret, making it difficult to gain insights from the model’s predictions.
  3. Resource Intensive: Training and using overfit models can be computationally expensive, as they have many parameters.

Underfitting: The Pitfall of Simplicity

What is Underfitting?

Underfitting occurs when a model is too simplistic to capture the underlying patterns in the data. In this case, the model fails to fit the training data properly and, consequently, performs poorly on both the training and test data.

Causes of Underfitting

Several factors can contribute to underfitting:

  1. Model Too Simple: If the model has too few parameters or is too shallow, it may not have the capacity to capture complex relationships in the data.
  2. Inadequate Features: If the features used to train the model are insufficient or not representative of the underlying data, the model may underfit.
  3. Noisy Data: Data with a high level of noise or measurement errors can make it challenging for any model to capture meaningful patterns.

Indicators of Underfitting

Recognizing underfitting is essential for model diagnosis. Indicators of underfitting include:

  • Low Training and Testing Accuracy: An underfit model will perform poorly on both the training and test datasets.
  • Simple Model Architecture: If your model is overly simple, such as a linear model for a highly nonlinear problem, it’s likely to underfit.
  • High Bias: High bias, as indicated by a substantial error on the training data, is a sign of underfitting.

Consequences of Underfitting

Underfitting also has its share of consequences:

  1. Poor Predictive Performance: Underfit models fail to capture the underlying patterns in the data, resulting in inaccurate predictions.
  2. Missed Opportunities: An underfit model might miss important relationships or insights present in the data, leading to missed opportunities for decision-making and discovery.
  3. Ineffective Decision Support: When models underfit, they can’t provide reliable decision support, which is a critical goal of many machine learning applications.

Striking the Balance: Model Complexity

The central challenge in machine learning is striking the right balance between model complexity and simplicity. This balance is often referred to as the “bias-variance trade-off.”

Bias: Bias refers to the error introduced by approximating a real-world problem, which may be complex, by a much simpler model. High bias can lead to underfitting.

Variance: Variance refers to the error introduced due to the model’s sensitivity to small fluctuations in the training data. High variance can lead to overfitting.

Achieving the right bias-variance balance involves finding a model complexity that fits the underlying patterns in the data without fitting the noise.

Mitigating Overfitting and Underfitting

Now that we understand the causes and consequences of overfitting and underfitting, let’s explore strategies to mitigate these issues.

1. Cross-Validation

Cross-validation is a technique for assessing how well a model will generalize to an independent dataset. It involves splitting the data into multiple subsets, training the model on different subsets, and evaluating its performance on the remaining data. This provides a more reliable estimate of a model’s true performance.

2. Regularization

Regularization techniques add a penalty term to the model’s objective function, discouraging overly complex models. Common forms of regularization include L1 regularization (Lasso) and L2 regularization (Ridge). These techniques constrain the model’s parameters and help prevent overfitting.

3. Feature Engineering

Feature engineering involves selecting and transforming the input features to make them more suitable for modeling. It can help reduce noise in the data and improve a model’s ability to capture meaningful patterns.

4. More Data

Increasing the amount of training data is often an effective way to combat overfitting. With more data, it becomes harder for a model to fit the noise in the data, leading to better generalization.

5. Model Complexity Control

Carefully selecting the appropriate model architecture and complexity is crucial. Sometimes, a simpler model with fewer parameters can outperform a complex one if it’s better suited to the problem.

6. Ensemble Learning

Ensemble methods, such as Random Forests and Gradient Boosting, combine multiple models to make more accurate predictions. These methods are robust against overfitting and often yield excellent results.

7. Early Stopping

In the training process, monitoring a model’s performance on a validation set and stopping when performance starts to degrade can prevent overfitting. This technique is known as early stopping.

8. Data Cleaning

Removing noisy or irrelevant data points can help improve a model’s ability to generalize. Data preprocessing techniques, such as outlier detection and imputation, are essential for data cleaning.

9. Model Interpretability

Simpler models are often more interpretable. If interpretability is a priority, consider using simpler model architectures.

10. Regular Monitoring

Even after implementing mitigation strategies, it’s essential to regularly monitor a model’s performance. Data distributions can change over time, potentially affecting a model’s performance.

The Ongoing Battle

The battle against overfitting and underfitting is ongoing in the field of machine learning. As new algorithms and techniques emerge, data scientists continue to refine their approaches to model development and evaluation. Moreover, understanding the nuances of a specific problem domain plays a critical role in achieving the right balance.

In conclusion, overfitting and underfitting are challenges that every data scientist and machine learning practitioner faces. Striking the balance between model complexity and simplicity is at the core of building models that generalize well to new data. By understanding the causes and consequences of these adversaries and employing appropriate mitigation strategies, we can create models that not only work but excel in solving real-world problems. Ultimately, mastering this balance is the key to successful data-driven decision-making and innovation.



Parth Sojitra

AI researcher and passionate blog writer exploring the frontiers of artificial intelligence and sharing insights with the world. 🤖✍️ #AI #Research