# How to measure classifier performance

Classification is a type of supervised machine learning problem where the goal is to predict the class or category of an input instance based on a set of features or attributes. In a classification problem, the target variable (i.e., the variable we want to predict) is categorical, meaning that it can take on a discrete set of values or labels.

There are two main types of classification problems: binary classification and multi-class classification.

Binary classification involves predicting one of two possible classes or labels, such as “spam” or “not spam”, “fraudulent” or “non-fraudulent”, or “positive” or “negative”. The goal of binary classification is to learn a model that can accurately distinguish between these two classes based on a set of input features.

Multi-class classification, on the other hand, involves predicting one of three or more possible classes or labels. For example, a multi-class classification problem might involve predicting the species of a plant based on its characteristics, with the possible classes being “rose”, “daisy”, “lily”, and so on.

Multi-class classification can be further divided into two subtypes: “exclusive” and “non-exclusive”. In exclusive multi-class classification, each instance can belong to only one class, while in non-exclusive multi-class classification, each instance can belong to multiple classes at the same time.

# Metrics

There are several metrics commonly used to evaluate the performance of classification models.

**Accuracy:**This measures the proportion of correct predictions made by the model, i.e., the number of true positives and true negatives divided by the total number of instances in the dataset.**Precision:**This measures the proportion of true positive predictions (i.e., the number of correct positive predictions) among all positive predictions made by the model. High precision means that the model makes few false positive predictions.**Recall:**This measures the proportion of true positive predictions among all actual positive instances in the dataset. High recall means that the model makes few false negative predictions.**F1 score:**This is the harmonic mean of precision and recall, and is a single number that combines both metrics. It is often used in situations where both precision and recall are important.**The area under the receiver operating characteristic curve (AUC-ROC):**This is a measure of how well the model can distinguish between positive and negative instances. It plots the true positive rate (recall) against the false positive rate, and the AUC-ROC score is the area under this curve. A score of 0.5 indicates that the model is no better than random, while a score of 1 indicates perfect performance.**Confusion matrix:**This is a table that shows the number of true positives, true negatives, false positives, and false negatives made by the model. It is a useful tool for visualizing the performance of the model and identifying areas for improvement.

# More about F1-Score

The F1 score is a commonly used metric for evaluating the performance of classification models. It is a single number that combines precision and recall, two key metrics used in binary classification problems.

Precision measures the proportion of true positive predictions (i.e., the number of correct positive predictions) among all positive predictions made by the model. Recall, on the other hand, measures the proportion of true positive predictions among all actual positive instances in the dataset.

The F1 score is the harmonic mean of precision and recall, and is calculated as follows:

`F1 score = 2 * (precision * recall) / (precision + recall)`

The F1 score ranges from 0 to 1, where a score of 1 indicates perfect precision and recall, while a score of 0 indicates that the model failed to make any correct positive predictions.

The F1 score is often used in situations where both precision and recall are important, as it provides a balanced view of the model’s performance. For example, in medical diagnosis, it is important to have both high precision (to minimize false positives) and high recall (to minimize false negatives).

In summary, the F1 score is a single number that combines precision and recall and is used to evaluate the performance of binary classification models.

# F1-score for Multi-class problems

When evaluating the performance of multi-class classification models, we need to modify the F1 score to account for the fact that there are more than two classes. There are several ways to do this, depending on how we want to weigh precision and recall for different classes.

- One common approach is to calculate the F1 score for each class separately, and then take the average of these scores to get an overall F1 score. This is called the “macro-averaged F1 score” and gives equal weight to each class, regardless of how many instances it has in the dataset.
- Another approach is to calculate the F1 score for each class separately, and then take a weighted average of these scores based on the number of instances in each class. This is called the “weighted F1 score” and gives more weight to classes with more instances, as they have a larger impact on the overall performance of the model.
- There are also other variations of the F1 score, such as the “micro-averaged F1 score” and the “samples F1 score”, which take different approaches to combining precision and recall across multiple classes.

# Conclusion

Classification is a type of supervised machine learning problem where the goal is to predict the class or category of an input instance based on a set of features or attributes. There are two main types of classification problems: binary classification and multi-class classification, which can be exclusive or non-exclusive. To evaluate the performance of classification models, several metrics are commonly used, including accuracy, precision, recall, F1 score, AUC-ROC, and confusion matrix. Choosing the appropriate metric depends on the specific requirements of the problem at hand.