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{{short description|Monte Carlo algorithm}}
[[
In [[statistics]] and [[statistical physics]], the '''Metropolis–Hastings algorithm''' is a [[Markov chain Monte Carlo]] (MCMC) method for obtaining a sequence of [[pseudo-random number sampling|random samples]] from a [[probability distribution]] from which direct sampling is difficult.
Metropolis–Hastings and other MCMC algorithms are generally used for sampling from multi-dimensional distributions, especially when the number of dimensions is high. For single-dimensional distributions, there are usually other methods (e.g. [[adaptive rejection sampling]]) that can directly return independent samples from the distribution, and these are free from the problem of [[autocorrelation|autocorrelated]] samples that is inherent in MCMC methods.
==History==
Some controversy exists with regard to credit for development of the algorithm. Metropolis, who was familiar with the computational aspects of the method, had coined the term "Monte Carlo" in an earlier article with [[Stanisław Ulam]], and led the group in the Theoretical Division that designed and built the [[MANIAC I]] computer used in the experiments in 1952. However, prior to 2003, there was no detailed account of the algorithm's development. Then, shortly before his death, [[Marshall Rosenbluth]] attended a 2003 conference at LANL marking the 50th anniversary of the 1953 publication. At this conference, Rosenbluth described the algorithm and its development in a presentation titled "Genesis of the Monte Carlo Algorithm for Statistical Mechanics".<ref name=Rosenbluth/> Further historical clarification is made by Gubernatis in a 2005 journal article<ref name=Gubernatis/> recounting the 50th anniversary conference. Rosenbluth makes it clear that he and his wife Arianna did the work, and that Metropolis played no role in the development other than providing computer time.▼
The algorithm is named in part for [[Nicholas Metropolis]], the first coauthor of a 1953 paper, entitled ''[[Equation of State Calculations by Fast Computing Machines]]'', with [[Arianna W. Rosenbluth]], [[Marshall Rosenbluth]], [[Augusta H. Teller]] and [[Edward Teller]]. For many years the algorithm was known simply as the ''Metropolis algorithm''.<ref>{{Cite book |last1=Kalos |first1=Malvin H. |title=Monte Carlo Methods Volume I: Basics |last2=Whitlock |first2=Paula A. |publisher=Wiley |year=1986 |___location=New York |pages=78–88}}</ref><ref>{{Cite journal |last=Tierney |first=Luke |date=1994 |title=Markov chains for exploring posterior distributions |url=https://projecteuclid.org/journals/annals-of-statistics/volume-22/issue-4/Markov-Chains-for-Exploring-Posterior-Distributions/10.1214/aos/1176325750.full |journal=The Annals of Statistics |volume=22 |issue=4 |pages=1701–1762|doi=10.1214/aos/1176325750 }}</ref> The paper proposed the algorithm for the case of symmetrical proposal distributions, but in 1970, [[W.K. Hastings]] extended it to the more general case.<ref name=Hastings/> The generalized method was eventually identified by both names, although the first use of the term "Metropolis-Hastings algorithm" is unclear.
▲Some controversy exists with regard to credit for development of the Metropolis algorithm.
This contradicts an account by Edward Teller, who states in his memoirs that the five authors of the 1953 article worked together for "days (and nights)".<ref name=Teller/> In contrast, the detailed account by Rosenbluth credits Teller with a crucial but early suggestion to "take advantage of statistical mechanics and take ensemble averages instead of following detailed kinematics". This, says Rosenbluth, started him thinking about the generalized Monte Carlo approach – a topic which he says he had discussed often with [[John von Neumann|John Von Neumann]]. Arianna Rosenbluth recounted (to Gubernatis in 2003) that Augusta Teller started the computer work, but that Arianna herself took it over and wrote the code from scratch. In an oral history recorded shortly before his death,<ref name=Barth/> Rosenbluth again credits Teller with posing the original problem, himself with solving it, and Arianna with programming the computer.▼
▲This contradicts an account by Edward Teller, who states in his memoirs that the five authors of the 1953 article worked together for "days (and nights)".<ref name=Teller/> In contrast, the detailed account by Rosenbluth credits Teller with a crucial but early suggestion to "take advantage of [[statistical mechanics]] and take ensemble averages instead of following detailed [[kinematics]]".
The Metropolis–Hastings algorithm can draw samples from any [[probability distribution]] with [[probability density]] <math>P(x)</math>, provided that we know a function <math>f(x)</math> proportional to the [[Probability density function|density]] <math>P</math> and the values of <math>f(x)</math> can be calculated. The requirement that <math>f(x)</math> must only be proportional to the density, rather than exactly equal to it, makes the Metropolis–Hastings algorithm particularly useful, because calculating the necessary normalization factor is often extremely difficult in practice.▼
==Description==
The Metropolis–Hastings algorithm works by generating a sequence of sample values in such a way that, as more and more sample values are produced, the distribution of values more closely approximates the desired distribution. These sample values are produced iteratively, with the distribution of the next sample being dependent only on the current sample value (thus making the sequence of samples into a [[Markov chain]]). Specifically, at each iteration, the algorithm picks a candidate for the next sample value based on the current sample value. Then, with some probability, the candidate is either accepted (in which case the candidate value is used in the next iteration) or rejected (in which case the candidate value is discarded, and current value is reused in the next iteration)—the probability of acceptance is determined by comparing the values of the function <math>f(x)</math> of the current and candidate sample values with respect to the desired distribution.▼
▲The Metropolis–Hastings algorithm can draw samples from any [[probability distribution]] with [[probability density]] <math>P(x)</math>, provided that we know a function <math>f(x)</math> proportional to the [[Probability density function|density]] <math>P</math> and the values of <math>f(x)</math> can be calculated. The requirement that <math>f(x)</math> must only be proportional to the density, rather than exactly equal to it, makes the Metropolis–Hastings algorithm particularly useful, because
▲The Metropolis–Hastings algorithm
For the purpose of illustration, the Metropolis algorithm, a special case of the Metropolis–Hastings algorithm where the proposal function is symmetric, is described below.
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general idea is to generate a sequence of samples which are linked in a [[Markov chain]]; in other words, where each sample in the sequence is conditionally independent of any earlier sample, given the sample immediately before it. The procedure for choosing successive samples guarantees that the distribution of sample values will match the desired distribution after a long time.!-->
Let <math>f(x)</math> be a function that is proportional to the desired probability density function <math>P(x)</math> (a.k.a. a target distribution){{efn|In the original paper by Metropolis et al. (1953), <math>f</math> was taken to be the [[Boltzmann distribution]] as the specific application considered was [[Monte Carlo integration]] of [[equation of state|equations of state]] in [[physical chemistry]]; the extension by Hastings generalized to an arbitrary distribution <math>f</math>.}}.
# Initialization: Choose an arbitrary point <math>x_t</math> to be the first observation in the sample and choose a proposal function <math>g(x\mid y)</math>. In this section, <math>g</math> is assumed to be symmetric; in other words, it must satisfy <math>g(x\mid y) = g(y\mid x)</math>.
▲# Initialization: Choose an arbitrary point <math>x_t</math> to be the first observation in the sample and choose an arbitrary probability density <math>g(x\mid y)</math> (sometimes written <math>Q(x\mid y)</math>) that suggests a candidate for the next sample value <math>x</math>, given the previous sample value <math>y</math>. In this section, <math>g</math> is assumed to be symmetric; in other words, it must satisfy <math>g(x\mid y) = g(y\mid x)</math>. A usual choice is to let <math>g(x\mid y)</math> be a [[Gaussian distribution]] centered at <math>y</math>, so that points closer to <math>y</math> are more likely to be visited next, making the sequence of samples into a [[random walk]]{{efn|In the original paper by Metropolis et al. (1953), <math>g(x\mid y)</math> was suggested to be a random translation with uniform density over some prescribed range.}}. The function <math>g</math> is referred to as the ''proposal density'' or ''jumping distribution''.
# For each iteration ''t'':
#* ''
#* ''Calculate'' the ''acceptance ratio'' <math>\alpha = f(x')/f(x_t)</math>, which will be used to decide whether to accept or reject the candidate{{efn|In the original paper by Metropolis et al. (1953), <math>f</math> was actually the [[Boltzmann distribution]], as it was applied to physical systems in the context of [[statistical mechanics]] (e.g., a maximal-entropy distribution of microstates for a given temperature at thermal equilibrium). Consequently, the acceptance ratio was itself an exponential of the difference in the parameters of the numerator and denominator of this ratio.}}. Because ''f'' is proportional to the density of ''P'', we have that <math>\alpha = f(x')/f(x_t) = P(x')/P(x_t)</math>.
#* ''Accept or reject'':
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#** If <math>u > \alpha</math>, then ''reject'' the candidate and set <math>x_{t+1} = x_t</math> instead.
This algorithm proceeds by randomly attempting to move about the sample space, sometimes accepting the moves and sometimes remaining in place. <math>P(x)</math> at specific point <math>x</math> is proportional to the iterations spent on the point by the algorithm. Note that the acceptance ratio <math>\alpha</math> indicates how probable the new proposed sample is with respect to the current sample, according to the distribution whose density is <math>P(x)</math>.
Compared with an algorithm like [[adaptive rejection sampling]]<ref name=":0">{{Cite journal |
* The samples are
* Although the Markov chain eventually converges to the desired distribution, the initial samples may follow a very different distribution, especially if the starting point is in a region of low density. As a result, a ''burn-in'' period is typically necessary,<ref>{{Cite book |title=Bayesian data analysis |date=2004 |publisher=Chapman & Hall / CRC |others=Gelman, Andrew |isbn=978-1584883883 |edition=2nd |___location=Boca Raton, Fla. |oclc=51991499}}</ref> where an initial number of samples are thrown away.
On the other hand, most simple [[rejection sampling]] methods suffer from the "[[curse of dimensionality]]", where the probability of rejection increases exponentially as a function of the number of dimensions.
In [[multivariate distribution|multivariate]] distributions, the classic Metropolis–Hastings algorithm as described above involves choosing a new multi-dimensional sample point.
==Formal derivation==
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## ''Increment'': set <math>t = t + 1</math>.
Provided that specified conditions are met, the empirical distribution of saved states <math>x_0, \ldots, x_T</math> will approach <math>P(x)</math>. The number of iterations (<math>T</math>) required to effectively estimate <math>P(x)</math> depends on the number of factors, including the relationship between <math>P(x)</math> and the proposal distribution and the desired accuracy of estimation.<ref>Raftery, Adrian E., and Steven Lewis. "How Many Iterations in the Gibbs Sampler?" ''In Bayesian Statistics 4''. 1992.</ref>
It is important to notice that it is not clear, in a general problem, which distribution <math>g(x' \mid x)</math> one should use or the number of iterations necessary for proper estimation; both are free parameters of the method, which must be adjusted to the particular problem in hand.
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==Step-by-step instructions==
[[
Suppose that the most recent value sampled is <math>x_t</math>. To follow the Metropolis–Hastings algorithm, we next draw a new proposal state <math>x'</math> with probability density <math>g(x' \mid x_t)</math> and calculate a value
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</math>
The Markov chain is started from an arbitrary initial value <math>x_0</math>, and the algorithm is run for many iterations until this initial state is "forgotten".
The algorithm works best if the proposal density matches the shape of the target distribution <math>P(x)</math>, from which direct sampling is difficult, that is <math>g(x' \mid x_t) \approx P(x')</math>.
If a Gaussian proposal density <math>g</math> is used, the variance parameter <math>\sigma^2</math> has to be tuned during the burn-in period.
This is usually done by calculating the ''acceptance rate'', which is the fraction of proposed samples that is accepted in a window of the last <math>N</math> samples.
The desired acceptance rate depends on the target distribution, however it has been shown theoretically that the ideal acceptance rate for a one-dimensional Gaussian distribution is about 50%, decreasing to about 23% for an <math>N</math>-dimensional Gaussian target distribution.<ref name=Roberts/> These guidelines can work well when sampling from sufficiently regular Bayesian posteriors as they often follow a multivariate normal distribution as can be established using the [[
If <math>\sigma^2</math>
if <math>\sigma^2</math> is too large, the acceptance rate will be very low because the proposals are likely to land in regions of much lower probability density, so <math>a_1</math> will be very small, and again the chain will converge very slowly. One typically tunes the proposal distribution so that the algorithms accepts on the order of 30% of all samples – in line with the theoretical estimates mentioned in the previous paragraph.
== Bayesian Inference ==
▲[[Image:3dRosenbrock.png|thumb|The result of three [[Markov chain]]s running on the 3D [[Rosenbrock function]] using the Metropolis–Hastings algorithm. The algorithm samples from regions where the [[posterior probability]] is high, and the chains begin to mix in these regions. The approximate position of the maximum has been illuminated. The red points are the ones that remain after the burn-in process. The earlier ones have been discarded.]]
{{main article|Bayesian Inference}}
MCMC can be used to draw samples from the [[posterior distribution]] of a statistical model.
The acceptance probability is given by:
<math>P_{acc}(\theta_i \to \theta^*)=\min\left(1, \frac{\mathcal{L}(y|\theta^*)P(\theta^*)}{\mathcal{L}(y|\theta_i)P(\theta_i)}\frac{Q(\theta_i|\theta^*)}{Q(\theta^*|\theta_i)}\right),</math>
where <math>\mathcal{L}</math> is the [[likelihood]], <math>P(\theta)</math> the prior probability density and <math>Q</math> the (conditional) proposal probability.
==See also==
* [[Genetic algorithm]]s
* [[Mean-field particle methods]]
* [[Metropolis light transport]]
* [[Multiple-try Metropolis]]
* [[Parallel tempering]]
* [[Particle filter|Sequential Monte Carlo]]
* [[Simulated annealing]]
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<ref name="Teller">Teller, Edward. ''Memoirs: A Twentieth-Century Journey in Science and Politics''. [[Perseus Publishing]], 2001, p. 328</ref>
<ref name="Barth">Rosenbluth, Marshall. [https://www.aip.org/history-programs/niels-bohr-library/oral-histories/28636-1 "Oral History Transcript"]. American Institute of Physics</ref>
<ref name="Gubernatis">{{Cite journal |last=J.E. Gubernatis |year=2005 |title=Marshall Rosenbluth and the Metropolis Algorithm |url=https://zenodo.org/record/1231899 |journal=[[Physics of Plasmas]] |volume=12 |issue=5 |
<ref name="Rosenbluth">{{Cite journal |last=M.N. Rosenbluth |year=2003 |title=Genesis of the Monte Carlo Algorithm for Statistical Mechanics |journal=[[AIP Conference Proceedings]] |volume=690 |pages=22–30 |bibcode=2003AIPC..690...22R |doi=10.1063/1.1632112}}</ref>
<!--<ref name="Dyson">{{Cite journal |last=F. Dyson |year=2006 |title=Marshall N. Rosenbluth |journal=[[Proceedings of the American Philosophical Society]] |volume=250 |pages=404}}</ref>-->
<ref name="Roberts">{{Cite journal |
<ref name="Roberts_Casella">{{Cite book |
<ref name="Newman_Barkema">{{Cite book |
}}
==Notes==
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== Further reading ==
* [[Bernd A. Berg]]. ''Markov Chain Monte Carlo Simulations and Their Statistical Analysis''. Singapore, [[World Scientific]], 2004.
*Chib,
* [http://www.tandfonline.com/doi/abs/10.1080/03610918.2013.777455#.VOk8J1PF9_c David D. L. Minh and Do Le Minh. "Understanding the Hastings Algorithm." Communications in Statistics - Simulation and Computation, 44:2
* Bolstad, William M. (2010) ''Understanding Computational Bayesian Statistics'', [[John Wiley & Sons]] {{ISBN|0-470-04609-0}}
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