Classification is the process of assigning a label to data points in a domain. The goal is to predict the class a new point will belong to, such as if it is a credit card application or medical test result. Classification is an important step in the analysis of information or data, but it can be a complex task. There are many techniques, and it is essential to understand the different metrics used to evaluate a classification model’s performance. Creating a classification scheme is not a one-time process; it must be regularly revisited to meet changing organizational needs or the discovery of emerging patterns in the data. This is especially true in domains where the resources are active, change frequently or have a probabilistic character such as software development or geopolitics. Supervised learning is the process of using a model to train a classification algorithm by feeding it data from the target domain.
The model learns from the data by recognizing patterns in it and using those patterns to identify similar patterns in new data sets. To create a classifier, a number of algorithms are available, including decision trees and support vector machines. These models look for patterns in the data and then use those patterns to predict the class a new point will be assigned to. Decision trees and SVMs are popular because they offer good performance and are easy to understand. A metric that evaluates the performance of a classification model is accuracy, which compares the expected outcome to the actual output. It is a very useful metric, but it should always be accompanied by additional metrics that offer more insight into the model’s performance and can help identify issues like class imbalance.
Class imbalance occurs when the classes in the data are not equally distributed. If, for example, 60% of the points in an image dataset belong to Apple and 40% to Mango, a classifier trained on that data will have trouble telling the difference between these two. In these situations, the ideal classification model would be one that achieves a perfect balance of recall and precision, but that is not practical in most cases. To address this problem, it is important to understand the concept of the confusion matrix, which shows how many observations fall into each of the classes. The best method to calculate a confusion matrix is by passing the ground truth and predicted values through a function that computes the difference between them.
There are also aggregate metrics that more fully summarize the confusion matrix, such as the F1 score or the Receiver Operating Characteristic (ROC) curve. Using any single metric, however, can be misleading without careful inspection of the underlying results. This is why it’s essential to understand the intended purpose of a classification system before choosing which metrics to evaluate it. If the resulting evaluations are not interpreted carefully, they can give false or even dangerous information. This is why it’s vital to examine the underlying results of any evaluations, especially when they are presented as tables.