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Overfitting

A modeling error that occurs when a function corresponds too closely to a particular set of data

What is Overfitting?

Overfitting is a term used in statistics that refers to a modeling error that occurs when a function corresponds too closely to a particular set of data. As a result, overfitting may fail to fit additional data, and this may affect the accuracy of predicting future observations.

 

Overfitting

 

Overfitting can be identified by checking validation metrics such as accuracy and loss. The validation metrics usually increase until a point where they stagnate or start declining when the model is affected by overfitting. During an upward trend, the model seeks a good fit, which, when achieved, causes the trend to start declining or stagnate.

 

Quick Summary

  • Overfitting is a modeling error that introduces bias to the model because it is too closely related to the data set.
  • Overfitting makes the model relevant to its data set only, and irrelevant to any other data sets.
  • Some of the methods used to prevent overfitting include ensembling, data augmentation, data simplification, and cross-validation.

 

How to Detect Overfitting?

Detecting overfitting is almost impossible before you test the data. It can help address the inherent characteristic of overfitting, which is the inability to generalize data sets. The data can, therefore, be separated into different subsets to make it easy for training and testing. The data is split into two main parts, i.e., a test set and a training set.

The training set represents a majority of the available data (about 80%), and it trains the model. The test set represents a small portion of the data set (about 20%), and it is used to test the accuracy of the data it never interacted with before. By segmenting the dataset, we can examine the performance of the model on each set of data to spot overfitting when it occurs, as well as see how the training process works.

The performance can be measured using the percentage of accuracy observed in both data sets to conclude on the presence of overfitting. If the model performs better on the training set than on the test set, it means that the model is likely overfitting.

 

How to Prevent Overfitting?

Below are some of the ways to prevent overfitting:

 

1. Training with more data

One of the ways to prevent overfitting is by training with more data. Such an option makes it easy for algorithms to detect the signal better to minimize errors. As the user feeds more training data into the model, it will be unable to overfit all the samples and will be forced to generalize to obtain results.

Users should continually collect more data as a way of increasing the accuracy of the model. However, this method is considered expensive, and, therefore, users should ensure that the data being used is relevant and clean.

 

2. Data augmentation

An alternative to training with more data is data augmentation, which is less expensive compared to the former. If you are unable to continually collect more data, you can make the available data sets appear diverse. Data augmentation makes a sample data look slightly different every time it is processed by the model. The process makes each data set appear unique to the model and prevents the model from learning the characteristics of the data sets.

Another option that works in the same way as data augmentation is adding noise to the input and output data. Adding noise to the input makes the model become stable, without affecting data quality and privacy, while adding noise to the output makes the data more diverse. However, noise addition should be done with moderation so that the extent of the noise is not so much as to make the data incorrect or too different.

 

3. Data Simplification

Overfitting can occur due to the complexity of a model, such that, even with large volumes of data, the model still manages to overfit the training dataset. The data simplification method is used to reduce overfitting by decreasing the complexity of the model to make it simple enough that it does not overfit.

Some of the actions that can be implemented include pruning a decision tree, reducing the number of parameters in a neural network, and using dropout on a neutral network. Simplifying the model can also make the model lighter and run faster.

 

4. Ensembling

Ensembling is a machine learning technique that works by combining predictions from two or more separate models. The most popular ensembling methods include boosting and bagging. Boosting works by using simple base models to increase their aggregate complexity. It trains a large number of weak learners arranged in a sequence, such that each learner in the sequence learns from the mistakes of the learner before it.

Boosting combines all the weak learners in the sequence to bring out one strong learner. The other ensembling method is bagging, which is the opposite of boosting. Bagging works by training a large number of strong learners arranged in a parallel pattern and then combining them to optimize their predictions.

 

More Resources

CFI is the official provider of the global Financial Modeling & Valuation Analyst (FMVA)™ certification program, designed to help anyone become a world-class financial analyst. To keep advancing your career, the additional CFI resources below will be useful:

  • Basic Statistics Concepts in Finance
  • Data-Mining Bias
  • Random Forest
  • Unconditional Probability

Financial Analyst Certification

Become a certified Financial Modeling and Valuation Analyst (FMVA)® by completing CFI’s online financial modeling classes and training program!