11.3 Bernoulli-Logit Generalised Linear Model (Logistic Regression)
Stan also supplies a single primitive for a Generalised Linear Model with Bernoulli likelihood and logit link function, i.e. a primitive for a logistic regression. This should provide a more efficient implementation of logistic regression than a manually written regression in terms of a Bernoulli likelihood and matrix multiplication.
11.3.1 Probability Mass Function
If \(x\in \mathbb{R}^{n\cdot m}, \alpha \in \mathbb{R}^n, \beta\in \mathbb{R}^m\), then for \(y \in {\{0,1\}}^n\), \[\begin{align*} &\text{BernoulliLogitGLM}(y~|~x, \alpha, \beta) = \prod_{1\leq i \leq n}\text{Bernoulli}(y_i~|~\text{logit}^{-1}(\alpha_i + x_i\cdot \beta))\\ &= \prod_{1\leq i \leq n} \left\{ \begin{array}{ll} \text{logit}^{-1}(\alpha_i + \sum_{1\leq j\leq m}x_{ij}\cdot \beta_j) & \text{if } y_i = 1, \text{ and} \\ 1 - \text{logit}^{-1}(\alpha_i + \sum_{1\leq j\leq m}x_{ij}\cdot \beta_j) & \text{if } y_i = 0. \end{array} \right. \end{align*}\]
11.3.2 Sampling Statement
y ~
bernoulli_logit_glm
(x, alpha, beta)
Increment target log probability density with bernoulli_logit_glm_lpmf(y | x, alpha, beta)
dropping constant additive terms.
11.3.3 Stan Functions
real
bernoulli_logit_glm_lpmf
(int[] y | matrix x, real alpha, vector beta)
The log Bernoulli probability mass of y given chance of success
inv_logit(alpha+x*beta)
, where a constant intercept alpha
is used
for all observations. The number of rows of the independent variable
matrix x
needs to match the length of the dependent variable vector
y
and the number of columns of x
needs to match the length of the
weight vector beta
.
real
bernoulli_logit_glm_lpmf
(int[] y | matrix x, vector alpha, vector beta)
The log Bernoulli probability mass of y given chance of success
inv_logit(alpha+x*beta)
, where an intercept alpha
is used that is
allowed to vary with the different observations. The number of rows of
the independent variable matrix x
needs to match the length of the
dependent variable vector y
and alpha
and the number of columns of
x
needs to match the length of the weight vector beta
.