# What is Recall in Machine Learning? Understanding the Basics of Recall Rates and Their Importance in Model Evaluation

Discover the power of recall in machine learning - the ability to correctly identify instances that belong to the positive class. Learn how recall differs from precision and F1 score, and why it’s a crucial metric for evaluating model performance.

Updated October 15, 2023

## Recall in Machine Learning: Understanding and Improving Accuracy

In machine learning, recall refers to the ability of a model to correctly identify all instances of a particular class or label. It is an important metric for evaluating the performance of a model, especially when the cost of false negatives (i.e., misclassifying a positive example as negative) is high. In this article, we will explore what recall is, how it is calculated, and techniques for improving recall in machine learning models.

## What is Recall in Machine Learning?

Recall, also known as true positive rate or sensitivity, measures the proportion of actual positive instances that are correctly identified by a model. It is defined as the number of true positives (TP) divided by the sum of true positives and false negatives (FN):

Recall = TP / (TP + FN)

A higher recall means that the model is more accurate at identifying positive instances, while a lower recall indicates that the model may be missing some positive instances.

Example:

Suppose we have a dataset of patients with a disease, and we want to build a machine learning model to predict whether a patient has the disease or not. If our model correctly identifies 90% of the patients who actually have the disease, but misses 10% of them, then our recall would be 90%.

## How to Calculate Recall?

To calculate recall, you need to know the number of true positives (TP) and false negatives (FN). TP is the number of instances that are actually positive and correctly identified by the model. FN is the number of instances that are actually positive but misclassified as negative by the model.

Here’s the formula to calculate recall:

Recall = TP / (TP + FN)

For example, let’s say we have a dataset with 100 instances, and our model correctly classifies 90 of them as positive, while misclassifying 10 as negative. The number of true positives (TP) is 90, and the number of false negatives (FN) is 10. To calculate recall, we would use the following formula:

Recall = 90 / (90 + 10) = 0.9 or 90%

## Techniques for Improving Recall

There are several techniques you can use to improve recall in your machine learning models:

### 1. Increase the sample size

The more data your model has to learn from, the better it will be at identifying positive instances. Increasing the sample size can help improve recall by providing more information for the model to learn from.

### 2. Use a different algorithm

Different machine learning algorithms have varying strengths and weaknesses when it comes to recall. For example, decision trees are good at identifying patterns in data, while support vector machines (SVMs) are better at separating classes. Experimenting with different algorithms can help you find one that is better suited to your dataset and task.

### 3. Use ensemble methods

Ensemble methods involve combining the predictions of multiple models to improve accuracy. Techniques such as bagging and boosting can help improve recall by reducing the variance of the model and combining the strengths of multiple models.

### 4. Use label smoothing

Label smoothing is a technique that involves adding noise to the labels in your dataset. This can help the model generalize better to new instances and improve recall.

### 5. Use data augmentation

Data augmentation involves generating additional training data by applying transformations to the existing data. This can help improve recall by providing more information for the model to learn from.

## Conclusion

Recall is an important metric for evaluating the performance of a machine learning model, especially when the cost of false negatives is high. By understanding what recall is, how it is calculated, and techniques for improving it, you can build more accurate models that are better at identifying positive instances. Remember to experiment with different algorithms, ensembles, and techniques to find the best approach for your dataset and task.