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In decision theory and estimation theory, a Bayes estimator is an estimator or decision rule that maximizes the posterior expected value of a utility function or minimizes the posterior expected value of a loss function (also called posterior expected loss). (See also prior probability.)

Specifically, suppose an unknown parameter θ is known to have a (proper) prior distribution $\Pi$. Let $\delta$ be an estimator of θ (based on some measurements), and let $R(\theta ,\delta )$ be a risk function, such as the mean squared error. The Bayes risk of $\delta$ is defined as $E_{\Pi }\{R(\theta ,\delta )\}$, where the expectation is taken over the probability distribution of $\theta$. An estimator $\delta$ is said to be a Bayes estimator if it minimizes the Bayes risk among all estimators. The estimator which minimizes the posterior expected loss for each x also minimizes the Bayes risk and therefore is a Bayes estimator.

If the prior is improper prior then an estimator which minimizes the posterior expected loss for each x is called Generalized Bayes estimator (or Generalized Bayes rule)

Examples

Risk functions are chosen depending on how one measures the distance between the estimate and the unknown parameter. Following are several examples of risk functions and the corresponding Bayes estimators. We denote the posterior generalized distribution function as $F$.

1. If we take the mean squared error as a risk function, then it is not difficult to show that the Bayes' estimate of the unknown parameter is simply the posterior mean,

   ${\widehat {\theta }}(x)=E[\theta |X]=\int \theta f(\theta |x)\,d\theta .$

   The Bayes risk, in this case, is the posterior variance.
2. A "linear" loss function, with $a>0$, which yields the posterior median as the Bayes' estimate:

   $L(\theta ,{\widehat {\theta }})=a|\theta -{\widehat {\theta }}|$

   $F({\widehat {\theta }}(x)|X)={\tfrac {1}{2}}$

3. Another "linear" loss function, which assigns different "weights" $a,b>0$ to over or sub estimation. It yields a quantile from the posterior distribution, and is a generalization of the previous loss function:

   $L(\theta ,{\widehat {\theta }})=\left\{{\begin{matrix}a|\theta -{\widehat {\theta }}|&{\mbox{for }}\theta -{\widehat {\theta }}\geq 0\\b|\theta -{\widehat {\theta }}|&\ \ \ {\mbox{for }}\theta -{\widehat {\theta }}<0\end{matrix}}\right.$

   $F({\widehat {\theta }}(x)|X)={\frac {a}{a+b}}$

4. The following loss function is trickier: it yields either the posterior mode, or a point close to it depending on the curvature and properties of the posterior distribution. Small values of the parameter $K>0$ are recommended, in order to use the mode as an approximation ($L>0$):

   $L(\theta ,{\widehat {\theta }})=\left\{{\begin{matrix}0&{\mbox{for }}|\theta -{\widehat {\theta }}|<K\\L&\ \ \ {\mbox{for }}|\theta -{\widehat {\theta }}|\geq K\end{matrix}}\right.$

Other loss functions can be conceived, although the mean squared error is the most widely used and validated.

Bayes estimators for conjugate priors

Using Conjugate prior makes the calculation of the posterior simple and makes the estimation process intuitive. It is specially useful for sequential estimation, where the posterior of the current iteration is used as the prior in the next iteration. Here are some examples:
If x|θ is normal x|θ~N(θ,σ²) and the prior is normal θ~N(μ,τ²) then the posterior is normal and the Bayes estimator under MSE is the posterior expectation,

${\widehat {\theta }}(x)={\frac {\sigma ^{2}}{\sigma ^{2}+\tau ^{2}}}\mu +{\frac {\tau ^{2}}{\sigma ^{2}+\tau ^{2}}}x$

If x₁,...,x_n are iid Poisson x_i|θ~P(θ) and the prior is Gamma θ~G(a,b) then the posterior is Gamma and the Bayes estimator under MSE is the posterior expectation,

${\widehat {\theta }}(X)={\frac {n{\overline {X}}+a}{n+{\frac {1}{b}}}}$

If x₁,...,x_n are iid Uniform x_i|θ~U(0,θ) and the prior is Pareto θ~Pa(θ₀,a) then the posterior is Pareto and the Bayes estimator under MSE is the posterior expectation,

${\widehat {\theta }}(X)={\frac {(a+n)\max {(\theta _{0},x_{1},...,x_{n})}}{a+n-1}}$

Generalized Bayes estimator

Improper prior has infinite mass $\int {\pi (\theta )d\theta }<\infty$ and as a result the Bayes risk is usually infinite and has no meaning. However, the posterior expected loss usually exists, represented by-

$\int {L(\theta ,a)\pi (\theta |x)d\theta }$

where L is the loss function, a is an action and π(θ|x) is the posterior density.
A Generalized Bayes estimator, for a given x, is an action which minimizes the posterior expected loss (when the prior π(θ) is improper).

A useful example is location parameter estimation under L(a-θ) loss fuction:
Here θ is a location parameter and f_x|θ=f(x-θ). It is common to use the improper prior π(θ)=1 in this case, specially when no other more subjective information is available. This yields,
π(θ|x)=π(θ)•f_x|θ=f(x-θ), so the posterior expected loss is (by defining y=x-θ),

$E[L(a-\theta )]=\int {L(a-\theta )f(x-\theta )d\theta }=\int {L(a-x+y)f(y)dy}$

Defining C=a-x we get,

$E[L(a-\theta )]=\int {L(C+y)f(y)dy}=E[L(y+C)]$

therefore the Generalized Bayes estimator is x+C where C is a constant minimizing E[L(y+C)].
Under MSE, as a private case, $C=E[y]=\int {yf(y)dy}$ and the generalized Bayes estimator is δ(x)=x-E[y].
Assuming for example gaussian samples X|θ~N(θ,I_p) where X=(x₁,...,x_p) and θ=(θ₁,...,θ_p) , then the generalized Bayes estimator of θ is δ(X)=X .

Empirical Bayes estimator

A Bayes estimator derived through Empirical Bayes method is called Empirical Bayes estimator. Empirical Bayes methods enable the use of auxiliary empirical data, from past observations, in the development of a Bayes estimator. This is under the assumption that the estimated parameters are from a common prior. Similarly, in compound decision problems (where simultaneous independent observations are being held) the data from current observations can be used.
Parametric empirical Bayes (PEB) is usually preferable since it is more applicable and more accurate on small amounts of data (see Berger , "Statistical decision theory and Bayesian analysis", section 4.5).

Example for PEB estimation:
Given x₁,...x_n past observations with the conditional distribution f(x_i|θ_i), the esimation of θ_n+1 based on x_n+1 is required.
Assuming that θ_i have common prior with a specific parametric form (e.g. normal), we can use the past observations to determine the moments of that prior μ_π and σ_π (mean and variance)in the following way:
First we estimate the moments μ_m and σ_m of the marginal distribution of x₁,...x_n by,

${\widehat {\mu }}_{m}={\frac {1}{n}}\sum {x_{i}}$

${\widehat {\sigma }}_{m}^{2}={\frac {1}{n}}\sum {(x_{i}-{\widehat {\mu }}_{m})^{2}}$

Then we can use the following connection, where μ_f(θ) and σ_f(θ) are the moments of the conditional distribution,

$\mu _{m}=E_{\pi }[\mu _{f}(\theta )],\sigma _{m}^{2}=E_{\pi }[\sigma _{f}^{2}(\theta )]+E_{\pi }[\mu _{f}(\theta )-\mu _{m}]$

Further assuming that μ_f(θ)=θ and σ_f(θ)=K is constant, we get:

$\mu _{\pi }=\mu _{m},\sigma _{\pi }^{2}=\sigma _{m}^{2}-\sigma _{f}^{2}=\sigma _{m}^{2}-K$

So finally we get the estimated momnets of the prior,

${\widehat {\mu }}_{\pi }={\widehat {\mu }}_{m}$

${\widehat {\sigma }}_{\pi }^{2}={\widehat {\sigma }}_{m}^{2}-K$

Now, if for example x_i|θ_i~N(θ_i,1) and we assume a normal prior (which is conjugate prior in this case) so $\theta _{n+1}\sim N({\widehat {\mu }}_{\pi },{\widehat {\sigma }}_{\pi }^{2})$ and we can calculate the Bayes estimator of θ_n+1 based on x_n+1.

Admissibility of Bayes estimators

Bayes rules with finite Bayes risk are typically admissible:

• If a Bayes rule is unique then it is admissible. For example, as stated above, under mean squared error (MSE) the Bayes rule is unique and therefore admissible.
• For discrete θ, Bayes rules are admissible.
• For continues θ, if the risk function R(θ,δ) is continues in θ for every δ then the Bayes rules are admissible.

However, Generalized Bayes rules usually have infinite Bayes risk. These can be inadmissible and the verification of thier admissibility can be difficult. For example, the generalized Bayes estimator of θ based on gaussian samples which is described in the "Generalized Bayes estimator" section above, is inadmissible for p>2 since it is well known that the James-Stein estimator has smaller risk for all θ.

Asymptotic efficiency of Bayes estimators

Suppose that x1,…,xn are iid samples with density f(xi|θ) and δ_n=δ(x1 ,…,xn) is Bayes estimator of θ. In addition, let $\theta _{0}\in \Theta$ be the true (unknown) value of θ. While the Bayesian analysis assumes θ has density π(θ) and posterior density π(θ|X), for analyzing the asymptotic behavior of δ we regard θ₀ as a deterministic parameter. Under specific conditions (see Lehmann and Casella, Theory of Point Estimation, section 6.8), for large sample (large values of n), the posterior density of θ is approximately normal. This means that for large n the effect of the prior probability which was given to θ declines!
Moreover, if δ is Bayes estimator under MSE then it is asymptotically unbiased and it converges in distribution to normal distribution:

${\sqrt {n}}(\delta _{n}-\theta _{0})\to N(0,{\frac {1}{I(\theta _{0})}})$

Where I(θ₀) is the fisher information of θ₀.
As a conclusion, the Bayes estimator δ_n under MSE is asymptotically efficient.

Another estimator which is asymptotically normal and efficient is the deterministic Maximum likelihood estimator (MLE), the relations between both (for large sample) can be shown in the following simple example:
Consider the estimator of θ based on binomial sample x~b(θ,n) where θ denotes the probability for success. Assuming the prior of θ is a Beta distribution, B(a,b), this is a conjugate prior and the posterior distribution is known to be B(a+x,b+n-x). So the Bayes estimator under MSE is,

$\delta _{n}(x)=E[\theta |x]={\frac {a+x}{a+b+n}}$

The MLE in this case is x/n and so we get,

$\delta _{n}(x)={\frac {a+b}{a+b+n}}E[\theta ]+{\frac {n}{a+b+n}}MLE$

The last equation implies that for n->∞ the Bayes estimator (in the described problem) is closed to the MLE. In the other hand when n is small the prior becomes more dominant.

See also

• Recursive Bayesian estimation
• Empirical Bayes method
• Conjugate prior

References

• Lehmann, E. L. (1998). Theory of Point Estimation. Springer. pp. 2nd ed. ISBN 0-387-98502-6. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Berger, J.O. (1985). Statistical Decision Theory and Bayesian Analysis. Springer Verlag, New York. pp. Second Edition. ISBN ISBN 0-387-96098-8 and also ISBN 3-540-96098-8. {{cite book}}: Check |isbn= value: invalid character (help)

External links

• Bayesian estimation on cnx.org