Composite classification metrics help you and other decision makers evaluate the quality of a model quickly. They often provide more valuable information than simple metrics such as recall, precision, or specificity.
Data scientists and statisticians should understand the most common composite classification metrics. This guide will help you keep them straight. 🎉
A flower is a composite. Source: pixabay.com
This is the third and final article in a series to help you understand, use, and remember the seven most popular classification metrics. In the first article in the series I explained the confusion matrix and the most common evaluation term: accuracy. In the second article I shined a light on the three most common basic metrics: recall (sensitivity), precision, and specificity. If you don’t have those terms down cold, I suggest you spend some more time with them before proceeding. 👍
Each of the composite metrics in this article is built from basic metrics. Let’s look at some beautiful composite metrics!
Balanced Accuracy
As you saw in the first article in the series, when outcome classes are imbalanced, accuracy can mislead.
Balanced accuracy is a better metric to use with imbalanced data. It accounts for both the positive and negative outcome classes and doesn’t mislead with imbalanced data.
Here’s the formula:
Balanced Accuracy_ = (((TP/(TP+FN)+(TN/(TN+FP))) / 2_
Thinking back to the last article, which metric is TP/(TP+FN) the formula for? That’s right, _recall _— also known as sensitivity and the true positive rate!
And which metric is TN/(TN+FP) the formula for? That’s right, _specificity, _also known as the true negative rate!
So here’s a shorter way to write the balanced accuracy formula:
Balanced Accuracy_ = (Sensitivity + Specificity) / 2_
Balanced accuracy is just the average of sensitivity and specificity. It’s great to use when they are equally important. ☝️
Let’s continue with an example from the previous articles in this series. Here are the results from our model’s predictions of whether a website visitor would purchase a shirt at Jeff’s Awesome Hawaiian Shirt store. 🌺👕
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