The list of interpretable models is constantly growing and of unknown size. It includes simple models such as linear models, decision trees and naive Bayes, but also more complex ones that combine or modify non-interpretable machine learning models to make them more interpretable. Especially publications on the latter type of models are currently being produced at high frequency and it is hard to keep up with developments. The book teases only the Naive Bayes classifier and k-nearest neighbors in this chapter.
The Naive Bayes classifier uses the Bayes’ theorem of conditional probabilities. For each feature, it calculates the probability for a class depending on the value of the feature. The Naive Bayes classifier calculates the class probabilities for each feature independently, which is equivalent to a strong (= naive) assumption of conditional independence of the features. Naive Bayes is a conditional probability model and models the probability of a class \(C_k\) as follows:
The term Z is a scaling parameter that ensures that the sum of probabilities for all classes is 1 (otherwise they would not be probabilities). The conditional probability of a class is the class probability times the probability of each feature given the class, normalized by Z. This formula can be derived by using the Bayes’ theorem.
Naive Bayes is an interpretable model because of the independence assumption. It can be interpreted on the modular level. It is very clear for each feature how much it contributes towards a certain class prediction, since we can interpret the conditional probability.
The k-nearest neighbor method can be used for regression and classification and uses the nearest neighbors of a data point for prediction. For classification, the k-nearest neighbor method assigns the most common class of the nearest neighbors of an instance. For regression, it takes the average of the outcome of the neighbors. The tricky parts are finding the right k and deciding how to measure the distance between instances, which ultimately defines the neighborhood.
The k-nearest neighbor model differs from the other interpretable models presented in this book because it is an instance-based learning algorithm. How can k-nearest neighbors be interpreted? First of all, there are no parameters to learn, so there is no interpretability on a modular level. Furthermore, there is a lack of global model interpretability because the model is inherently local and there are no global weights or structures explicitly learned. Maybe it is interpretable at the local level? To explain a prediction, you can always retrieve the k neighbors that were used for the prediction. Whether the model is interpretable depends solely on the question whether you can ‘interpret’ a single instance in the dataset. If an instance consists of hundreds or thousands of features, then it is not interpretable, I would argue. But if you have few features or a way to reduce your instance to the most important features, presenting the k-nearest neighbors can give you good explanations.