# Basic of Statistical Learning

Posted by GwanSiu on May 6, 2020

## 1. The Bayes Decision Rule for Minimum Error

• The a-postieriori probability of a sample
$\begin{equation} P(Y=i\vert X) = \frac{p(X\vert Y=i)P(Y=i)}{P(X)}=\frac{\pi_{i}p_{i}(X\vert Y=i)}{\sum_{i=1}\pi_{i}p_{i}(X\vert Y=i)} = q_{i}(X) \end{equation}$
• Bayes Test:
$\begin{equation} \begin{split} \frac{q_{1}(x)}{q_{2}(x)} &\lesseqgtr 1 \\ \Reftarrow\frac{\pi_{1}p_{1}(X\vert Y=1)}{\pi_{2}p_{2}(X\vert Y=2)}&\lesseqgtr 1 \\ \Rightarrow \frac{p_{1}}{p_{2}}&\lesseqgtr \frac{\pi_{2}}{\pi_{1}} \end{split} \end{equation}$
• Likelihood ratio:
$\begin{equation} \ell(X) = \frac{p_{1}}{p_{2}} \end{equation}$
• Discriminant function:
$\begin{equation} h(x)=\log \ell(x)=\log(p_{1})-\log(p_{2})\lesseqgtr \log(\pi_{1})-\log(\pi_{2}) \end{equation}$

## 2. Bayer Error

In machine learning, we should compute the probability of error so as to decide whether a calssifier is good or not.

Probability eorr: the probability that a sample is assigned to the wrong class. Given a datum $X$, the risk is defined as follows:

$\begin{equation} r(X)=min(q_{1}(X), q_{2}(X)) \end{equation}$

The bayes error (the expected risk):

$\begin{equation} \begin{split} \epsilon &= \mathbb{E}(r(X))=\int r(x)p(x)\mathrm{d}x\\ &=\int \min(\pi p_{1}(x), \pi_{2}p_{2})\mathrm{d}x \\ &=\pi_{1}\int_{L1}p_{1}(x)\mathrm{d}x +\pi_{2}\int_{L2}p_{2}(x)\mathrm{d}x \\ &=\pi_{1}\epsilon_{1} + \pi_{2}\epsilon_{2} \end{split} \end{equation}$ 