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What is Underfitting?

Underfitting is a common problem encountered in Machine Learning, where a model is unable to capture the underlying patterns in the data due to its simplicity. This can lead to poor performance on both the training and test data. In this article, we will explore the concept in more detail, its causes, and the techniques that can be used to address it. We will also discuss the importance of avoiding underfitting in Machine Learning applications.

What is Underfitting and what are the effects of it?

Underfitting is a concept in Machine Learning that occurs when a model is unable to capture the underlying patterns and relationships in the data, resulting in poor predictive performance. It happens when the model is too simplistic or lacks the necessary complexity to represent the true nature of the data.

When a model underfits, it fails to learn the intricate details and nuances present in the dataset. It may oversimplify the relationships between the input features and the target variable, leading to inaccurate predictions. Underfitting is often characterized by high bias, which means the model makes overly generalized assumptions about the data.

One of the primary effects of underfitting is poor predictive performance. The model fails to capture the complexity of the data, resulting in inaccurate predictions on both the training and test datasets. This lack of accuracy limits the model’s usefulness in real-world applications.

Underfitting is often associated with high bias. The model makes oversimplified assumptions about the data, leading to consistent under- or over-predictions. This bias prevents the model from accurately representing the true relationships between the input features and the target variable.

Another effect of underfitting is the inability to generalize well to unseen data. The model struggles to capture the full range of patterns and variations present in the training data, resulting in poor performance when applied to new, unseen examples. This lack of generalization limits the model’s usefulness in real-world scenarios.

Additionally, underfitting is characterized by limited learning capacity. The model is too simplistic to learn and incorporate complex relationships present in the data. It fails to utilize the available information effectively, leading to missed opportunities for accurate predictions and valuable insights.

Underfit models also exhibit reduced flexibility. They lack the capacity to adapt to variations and complexities in the data, resulting in suboptimal performance. The model’s simplicity prevents it from capturing the full range of patterns and interactions, limiting its ability to make accurate predictions.

Why does Underfitting occur?

Underfitting occurs when a Machine Learning model is unable to capture the underlying patterns in the data, resulting in poor performance on both the training and test sets. There are several reasons why underfitting can occur, including:

  1. Model complexity: If the model is too simple, it may not have enough capacity to learn the patterns in the data. This can happen when the model has too few parameters or features.
  2. Insufficient training: If the model is not trained for long enough or with enough data, it may not be able to capture the underlying patterns in the data.
  3. Inappropriate model selection: If the model is not appropriate for the type of data being used, it may not be able to capture the underlying patterns in the data.
  4. Incorrect preprocessing: If the data is not preprocessed correctly, it may contain noise or irrelevant features that can confuse the model, leading to underfitting.

Overall, underfitting is a common problem in Machine Learning that can be caused by a variety of factors. Addressing these factors can help improve model performance and avoid the problem.

What are examples of Underfitting in Machine Learning models?

Underfitting occurs when a Machine Learning model is too simple to capture the underlying patterns in the data. In such cases, the model is not able to learn the relevant patterns and as a result, the model’s performance on the training set as well as on unseen data is poor. Here are some common examples in Machine Learning models:

  1. Linear models: Linear models assume a linear relationship between the input features and the output variable. If the true relationship is not linear, the model may underfit the data and perform poorly.
  2. Decision trees: Decision trees can be prone to underfitting if they are too shallow or if the tree is not complex enough to capture the underlying patterns in the data.
  3. Neural networks: Neural networks can also suffer from underfitting if they are not complex enough. If the network is too small or has too few hidden layers, it may not be able to capture the non-linear relationships in the data.
  4. Support vector machines: Support vector machines can underfit the data if the kernel function used is not appropriate for the data.

In all of these cases, the model is too simple to capture the complexity of the data, leading to underfitting.

What is the difference between Underfitting and Overfitting?

In Machine Learning, both underfitting and overfitting are common problems that can affect the performance of a model. While underfitting occurs when a model is too simple and fails to capture the underlying patterns in the data, overfitting occurs when a model is too complex and captures noise in the data, resulting in poor generalization performance.

Underfitting can be thought of as a model that is not able to learn from the data as well as it could. This can occur for a variety of reasons, such as using a model that is too simple or using too few training examples. As a result, an underfit model may have poor accuracy on the training data and is likely to perform poorly on new, unseen data as well.

On the other hand, overfitting occurs when a model becomes too complex and starts to fit the noise in the training data rather than the underlying patterns. This can occur when a model is too flexible or when it has too many features relative to the number of training examples. An overfit model may perform very well on the training data but will likely have poor performance on new, unseen data.

Overfitting vs Underfitting
Differences between Underfitting, Overfitting, and Generalisation | Source: Author

To determine whether a model is underfitting or overfitting, it is important to evaluate its performance on both the training and validation sets. If the model has a high error on the training set, it may be underfitting, and if it has a high error on the validation set, it may be overfitting.

To address underfitting, it may be necessary to increase the complexity of the model or add more features to it. To address overfitting, regularization techniques such as L1 and L2 regularization can be used to penalize large weights and reduce the model’s complexity. Other techniques such as early stopping, dropout, and pruning can also be used to prevent overfitting.

In summary, underfitting and overfitting are common problems in machine learning that can affect the performance of a model. While underfitting occurs when a model is too simple and cannot capture the underlying patterns in the data, overfitting occurs when a model is too complex and captures noise in the data. To address these issues, it is important to carefully evaluate the model’s performance on both the training and validation sets and use appropriate techniques such as regularization to adjust the model’s complexity.

How to detect underfitting in models?

Detecting underfitting in Machine Learning models is essential to improve the model’s performance. One of the easiest ways to detect it is to observe the model’s performance on the training set and the validation set. If the model performs poorly on both sets, it is likely that the model underfits the data. The following are some of the common methods used to detect underfitting in models:

  • Performance Metrics: Performance metrics such as accuracy, precision, recall, F1-score, and the AUC-ROC curve can be used to evaluate the model’s performance. If the performance metrics are low, it may indicate that the model is underfitting the data.
ROC Curve Diagram
Example of a ROC – Curve | Source: Author
  • Learning Curves: Learning curves are graphs that show the model’s performance on the training and validation sets as the sample size increases. If the model is underfitting, the learning curves will converge to a low score for both sets.
  • Residual Plots: Residual plots are used to visualize the difference between the actual and predicted values of the dependent variable. If the residual plot shows a random scatter around zero, it indicates that the model is a good fit. However, if there is a pattern in the residual plot, it may indicate that the model is underfitting.
  • Feature Importance: Feature importance measures the contribution of each feature in the model. If some features have very low importance, it may indicate that the model is underfitting and not capturing the important features in the data.

Detecting underfitting in models is important as it helps identify the areas where the model needs improvement. Once it is detected, the model can be fine-tuned by adding more features, increasing the model’s complexity, or using a different algorithm to improve the model’s performance.

What are techniques for preventing Underfitting?

To prevent underfitting, it is necessary to increase the complexity of the model. Here are some techniques to prevent it:

  1. Adding more features: Adding more relevant features to the model can increase the complexity of the model, and provide more information for the model to make predictions.
  2. Increase model complexity: If the model is too simple and is not able to capture the patterns in the data, then increasing the model’s complexity can help in preventing underfitting. This can be done by increasing the number of layers in a neural network, increasing the number of decision trees in a random forest, or increasing the degree of polynomial functions used in linear regression.
  3. Reduce regularization: Regularization techniques such as L1 and L2 can prevent overfitting by adding a penalty term to the loss function. However, in some cases, too much regularization can lead to underfitting. Reducing the regularization strength or completely removing it can help prevent the problem.
  4. Increasing the amount of training data: Increasing the amount of training data can help in preventing underfitting by providing more information to the model. With more data, the model can better capture the underlying patterns in the data.
  5. Ensembling: Combining multiple models can help in preventing underfitting. Ensemble techniques such as bagging, boosting, and stacking can be used to combine the predictions of multiple models. This can help in increasing the complexity of the model.

Overall, it is important to strike a balance between model complexity and overfitting. With the right techniques and careful tuning of hyperparameters, underfitting can be prevented in Machine Learning models.

This is what you should take with you

  • Underfitting occurs when a machine learning model is too simple and cannot capture the complexity of the data, resulting in poor performance on both training and testing data.
  • It can be caused by various factors, such as a lack of features, an oversimplified model architecture, or insufficient training time.
  • This problem can be detected by observing the performance of the model on the training and testing data and comparing it to the expected performance.
  • To prevent underfitting, various techniques can be used, such as increasing the model’s complexity, adding more features, using a more powerful model architecture, or increasing the training time.
  • However, it’s important to note that increasing the model’s complexity should be done carefully, as it may lead to overfitting, which is another problem in machine learning.
  • Balancing the model’s complexity with the available data and the desired performance is an important aspect of machine learning, and understanding underfitting is a key step in achieving this balance.
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Other Articles on the Topic of Underfitting

Scikit-Learn has an interesting article on the differences between Overfitting and Underfitting.

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