# Statistical decision theory¶

Work in progress, initially just copying over from Wikipedia article: Admissible decision rule

Define sets $$\Theta$$, $${\mathcal {X}}$$, and $${\mathcal {A}}$$, where

• $$\Theta$$ are the states of nature,

• $${\mathcal {X}}$$ the possible observations, and

• $${\mathcal {A}}$$ the actions that may be taken.

An observation $$x\in {\mathcal {X}}$$ is distributed as $$F(x\mid \theta )$$ and therefore provides evidence about the state of nature $$\theta \in \Theta$$.

A decision rule is a function $$\delta :{{\mathcal {X}}}\rightarrow {{\mathcal {A}}}$$, where upon observing $$x\in {\mathcal {X}}$$, we choose to take action $$\delta (x)\in {\mathcal {A}}$$.

Also define a loss function $$L:\Theta \times {\mathcal {A}}\rightarrow {\mathbb {R}}$$, which specifies the loss we would incur by taking action $$a\in {\mathcal {A}}$$ when the true state of nature is $$\theta \in \Theta$$. Usually we will take this action after observing data $$x\in {\mathcal {X}}$$, so that the loss will be $$L(\theta ,\delta (x))$$. (It is possible though unconventional to recast the following definitions in terms of a utility function, which is the negative of the loss.)

Define the risk function as the expectation $$R(\theta ,\delta )=\operatorname {E}_{{F(x\mid \theta )}}[{L(\theta ,\delta (x))]}.\,\!$$

Whether a decision rule $$\delta\,\!$$ has low risk depends on the true state of nature $$\theta$$. A decision rule $$\delta ^{*}$$ dominates a decision rule $$\delta$$ if and only if $$R(\theta ,\delta ^{*})\leq R(\theta ,\delta )$$ for all $$\theta$$, and the inequality is strict for some $$\theta$$.

## Bayes rules:¶

Let $$\pi (\theta )$$ be a probability distribution on the states of nature. From a Bayesian point of view, we would regard it as a prior distribution. That is, it is our believed probability distribution on the states of nature, prior to observing data. For a frequentist, it is merely a function on $$\Theta$$ with no such special interpretation. The Bayes risk of the decision rule $$\delta$$ with respect to $$\pi (\theta )$$ is the expectation

(32)$$$r(\pi ,\delta )=\operatorname {E}_{{\pi (\theta )}}[R(\theta ,\delta )].$$$

A decision rule $$\delta$$ that minimizes $$r(\pi ,\delta )$$ is called a Bayes rule with respect to $$\pi (\theta )$$. There may be more than one such Bayes rule.

## Generalized Bayes rules:¶

In the Bayesian approach to decision theory, the observed $$x$$ is considered fixed. Whereas the frequentist approach (i.e., risk) averages over possible samples $$x\in {\mathcal {X}}$$ the Bayesian would fix the observed sample $$x$$ and average over hypotheses $$\theta \in \Theta$$. Thus, the Bayesian approach is to consider for our observed $$x$$ the expected loss.

(33)$$$\rho (\pi ,\delta \mid x)=\operatorname {E}_{{\pi (\theta \mid x)}}[L(\theta ,\delta (x))]$$$

where the expectation is over the posterior of $$\theta$$ given $$x$$ (obtained from $$\pi (\theta )$$ and $$F(x\mid \theta )$$ using Bayes’ theorem).

Having made explicit the expected loss for each given $$x$$ separately, we can define a decision rule $$\delta$$ by specifying for each $$x$$ an action $$\delta (x)$$ that minimizes the expected loss. This is known as a generalized Bayes rule with respect to $$\pi (\theta )$$. There may be more than one generalized Bayes rule, since there may be multiple choices of $$\delta (x)$$ that achieve the same expected loss.

According to the complete class theorems, under mild conditions every admissible rule is a (generalized) Bayes rule (with respect to \textit{some} prior $$\pi (\theta )$$ —- possibly an improper one -— that favors distributions $$\theta$$ where that rule achieves low risk). Thus, in frequentist decision theory it is sufficient to consider only (generalized) Bayes rules.