---
title: "Generalized Linear Model"
type: concept
related: [Gaussian Distribution, Generalized Linear Models, Exponential Family, Probability Of K In X Time, Exponential Distribution]
source: https://www.jemoka.com/posts/kbhgeneralized_linear_models/
confidence: high
status: active
---

A Generalized Linear Model is a model of data with the following properties:
The model for \(P(y \mid x; \theta)\) should come from a exponential family (depending on what your distribution of \(y\) is&mdash;for real data, we pick Gaussian distribution, for binary data, we pick Bernoulli distribution, for counts, we use poisson distribution, \(\mathbb{R}^{+}\) we use gamma distribution or exponential distribution, and for distributions of distributions we use Beta Distribution or Dirichlet Distribution). \(\eta = \theta^{T}x\), where \(\theta,x \in \mathbb{R}^{d}\) at test time&hellip; we want to output \(\mathbb{E}\qty [y|x; \theta]\) so our predictor is written as \(h_{\theta}\qty(x) = \mathbb{E}\qty [y|x; \theta]\) at train time, we maximize log likelihood \(\max_{\theta} \sum_{i=1}^{n} \log P\qty(y^{(i)} | \theta^{T}x^{(i)})\) to update using gradient ascend, \(\theta_{j} = \theta_{j} + \alpha \sum_{i=1}^{n} \qty(y^{(i)} - h_{\theta}\qty(x^{(i)}))x_{j}^{(i)}\) We also have two fancy names for things
components of exponential distribution canonical response function \begin{equation} \mu = \mathbb{E}[y|\eta] = g\qty(\eta) \end{equation}
canonical link function \begin{equation} \eta = g^{-1}\qty(\mu) \end{equation}
expectation \begin{equation} g\qty(\eta) = \pdv \eta a\qty(\eta) \end{equation}
example assume \(P(y|x; \theta) \sim \text{ExpFam}\qty(\eta)\) , where \(\eta = \theta^{T}x\) (since Bernoulli distribution is in the exponential family). Recall \(P\qty(y=1|x, \theta) = \phi = \frac{1}{1+e^{-\eta}} = \frac{1}{1+e^{-\theta^{\top}X}}\)
