Linear discriminant analysis

Linear discriminant analysis (LDA) is a linear classification method. Logistic regression directly models \(P(y \mid x)\). From Bayes’ Rule, we get \(P(y \mid x) = \frac{P(x \mid y) P(y)}{P(x)}\). Instead of modeling \(P(y \mid x)\) directly, LDA models \(P(x, y) = P(x \mid y)P(y)\) (\(P(x)\) doesn’t depend on the class). In particular, it assumes that the conditional distribution of \(x\) given \(y\) is a multivariate Gaussian. With a training dataset, we can estimate the mean and the covariance of the Gaussian for \(y = 1\) and the mean and the covariance for \(y = 0\). We set the densities equal to find the decision boundary that separates the classes. Different assumptions about the covariance matrices lead to different methods:

Covariance matrices

Different across classes

Same across classes



Diagonal QDA


Naive Bayes

Diagonal LDA


Spherical QDA

Nearest centroid

Note that LDA makes a distribution assumption about the features, while logistic regression makes no such assumption.