# What is Recall in Machine Learning?

Machine learning is a powerful technology that enables computers to learn from data and make decisions or predictions based on that learning. One essential aspect of evaluating the performance of machine learning models is “recall.”

Recall is a metric used to measure how well a model can correctly identify positive instances from the total number of positive instances present in the data. In this article, we will delve into what recall means, how it is calculated, and why it is crucial in assessing the effectiveness of machine learning models.

## Understanding Recall

In a binary classification problem, where the goal is to classify data into two categories (e.g., spam or non-spam emails, diseased or healthy patients), we have four possible outcomes for each data point: true positive (TP), false positive (FP), true negative (TN), and false negative (FN).

• True Positive (TP): The model correctly predicts a positive instance as positive.
• False Positive (FP): The model incorrectly predicts a negative instance as positive.
• True Negative (TN): The model correctly predicts a negative instance as negative.
• False Negative (FN): The model incorrectly predicts a positive instance as negative.

Recall focuses specifically on the true positive and false negative outcomes. It measures the ability of the model to identify all positive instances correctly. High recall indicates that the model is effective at minimizing false negatives and capturing most of the positive instances, while low recall suggests the model is missing a significant number of positive cases.

## Calculating Recall

Recall is calculated using the following formula:

``Recall = TP / (TP + FN)``

Here, TP represents the number of true positive predictions, and FN represents the number of false negatives.

The recall value ranges from 0 to 1, where 1 indicates perfect recall, meaning the model correctly identified all positive instances, and 0 indicates complete failure to detect positive instances.

## Importance of Recall in Machine Learning

Recall is a critical metric, especially in scenarios where detecting positive instances is crucial, such as in medical diagnoses, fraud detection, or identifying defective products on an assembly line. In these cases, false negatives can have severe consequences, and we want to ensure the model minimizes the number of such errors.

For example, in a cancer diagnosis model, false negatives would mean that a patient with cancer might be misdiagnosed as healthy, leading to delayed treatment and potential harm to the patient. Thus, high recall in this scenario would be a top priority to minimize such false negatives.

## Balancing Recall and Precision

Recall is often in competition with another important metric called precision. Precision measures how many of the positively classified instances are actually correct. Balancing recall and precision is crucial as there’s usually a trade-off between the two. Improving recall might increase false positives, reducing precision, and vice versa.

• High Recall, Low Precision: The model captures most positive instances but might also produce many false positives.
• Low Recall, High Precision: The model misses some positive instances but is more confident in the ones it predicts as positive.

The choice between optimizing for recall or precision depends on the specific use case and its requirements. In certain applications, striking a balance might be the most suitable approach.

## Conclusion

Recall is an essential metric in machine learning that measures a model’s ability to correctly identify positive instances. It plays a crucial role in applications where false negatives can have severe consequences. Understanding the balance between recall and precision is vital in building effective machine learning models that cater to the specific needs of the problem domain.

By evaluating the recall of a model, data scientists can fine-tune their algorithms, optimize for specific use cases, and make more informed decisions about the model’s suitability for real-world applications.

## Online Resources and References

1. Scikit-learn Documentation – Precision, Recall, and F1-Score
This page from the Scikit-learn documentation provides an in-depth explanation of precision, recall, and F1-score, along with practical examples.
2. Towards Data Science – Understanding Recall, Precision, and F1-Score
This blog post on Towards Data Science delves into the concepts of recall, precision, and F1-score, with clear explanations and examples.
3. Machine Learning Mastery – How to Calculate Precision, Recall, F1, and More
This article from Machine Learning Mastery provides a comprehensive guide on calculating precision, recall, F1-score, and confusion matrices.
4. Towards AI – Evaluating Machine Learning Models: A Beginner’s Guide
This beginner-friendly guide from Towards AI covers the evaluation metrics, including recall, for assessing machine learning models.