marginalLikelihood {BayesianTools}R Documentation

Calcluated the marginal likelihood from a set of MCMC samples

Description

Calcluated the marginal likelihood from a set of MCMC samples

Usage

marginalLikelihood(sampler, numSamples = 1000, method = "Chib", ...)

Arguments

sampler

an MCMC or SMC sampler or list, or for method "Prior" also a BayesianSetup

numSamples

number of samples to use. How this works, and if it requires recalculating the likelihood, depends on the method

method

method to choose. Currently available are "Chib" (default), the harmonic mean "HM", sampling from the prior "Prior", and bridge sampling "Bridge". See details

...

further arguments passed to getSample

Details

The marginal likelihood is the average likelihood across the prior space. It is used, for example, for Bayesian model selection and model averaging.

It is defined as

ML = \int L(Θ) p(Θ) dΘ

Given that MLs are calculated for each model, you can get posterior weights (for model selection and/or model averaging) on the model by

P(M_i|D) = ML_i * p(M_i) / (∑_i ML_i * p(M_i) )

In BT, we return the log ML, so you will have to exp all values for this formula.

It is well-known that the ML is VERY dependent on the prior, and in particular the choice of the width of uninformative priors may have major impacts on the relative weights of the models. It has therefore been suggested to not use the ML for model averaging / selection on uninformative priors. If you have no informative priors, and option is to split the data into two parts, use one part to generate informative priors for the model, and the second part for the model selection. See help for an example.

The marginalLikelihood function currently implements four ways to calculate the marginal likelihood. Be aware that marginal likelihood calculations are notoriously prone to numerical stability issues. Especially in high-dimensional parameter spaces, there is no guarantee that any of the implemented algorithms will converge reasonably fast. The recommended (and default) method is the method "Chib" (Chib and Jeliazkov, 2001), which is based on MCMC samples, with a limited number of additional calculations. Despite being the current recommendation, note there are some numeric issues with this algorithm that may limit reliability for larger dimensions.

The harmonic mean approximation, is implemented only for comparison. Note that the method is numerically unrealiable and usually should not be used.

The third method is simply sampling from the prior. While in principle unbiased, it will only converge for a large number of samples, and is therefore numerically inefficient.

The Bridge method uses bridge sampling as implemented in the R package "bridgesampling". It is potentially more exact than the Chib method, but might require more computation time. However, this may be very dependent on the sampler.

Value

A list with log of the marginal likelihood, as well as other diagnostics depending on the chose method

Author(s)

Florian Hartig

References

Chib, Siddhartha, and Ivan Jeliazkov. "Marginal likelihood from the Metropolis-Hastings output." Journal of the American Statistical Association 96.453 (2001): 270-281.

Dormann et al. 2018. Model averaging in ecology: a review of Bayesian, information-theoretic, and tactical approaches for predictive inference. Ecological Monographs

See Also

WAIC, DIC, MAP

Examples

##############################################################
# Comparison of ML for two regression models

# Creating test data with quadratic relationship
sampleSize = 30
x <- (-(sampleSize-1)/2):((sampleSize-1)/2)
y <-  1 * x + 1*x^2 + rnorm(n=sampleSize,mean=0,sd=10)
# plot(x,y, main="Test Data")

# likelihoods for linear and quadratic model 
likelihood1 <- function(param){
  pred = param[1] + param[2]*x + param[3] * x^2
  singlelikelihoods = dnorm(y, mean = pred, sd = 1/(param[4]^2), log = TRUE)
  return(sum(singlelikelihoods))  
}
likelihood2 <- function(param){
  pred = param[1] + param[2]*x 
  singlelikelihoods = dnorm(y, mean = pred, sd = 1/(param[3]^2), log = TRUE)
  return(sum(singlelikelihoods))  
}

setUp1 <- createBayesianSetup(likelihood1, 
                              lower = c(-5,-5,-5,0.01), 
                              upper = c(5,5,5,30))
setUp2 <- createBayesianSetup(likelihood2, 
                              lower = c(-5,-5,0.01), 
                              upper = c(5,5,30))

out1 <- runMCMC(bayesianSetup = setUp1)
M1 = marginalLikelihood(out1, start = 1000)

out2 <- runMCMC(bayesianSetup = setUp2)
M2 = marginalLikelihood(out2, start = 1000)


### Calculating Bayes factor

exp(M1$ln.ML - M2$ln.ML)

# BF > 1 means the evidence is in favor of M1. See Kass, R. E. & Raftery, A. E. 
# (1995) Bayes Factors. J. Am. Stat. Assoc., Amer Statist Assn, 90, 773-795.

### Calculating Posterior weights

exp(M1$ln.ML) / ( exp(M1$ln.ML) + exp(M2$ln.ML))

# If models have different model priors, multiply with the prior probabilities of each model. 


## Not run:  
#############################################################
# Fractional Bayes factor

# Motivation: ML is very dependent on the prior, which is a problem if you 
# have uninformative priors. you can see this via rerunning the upper  
# example with changed priors - suddenly, support for M1 is gone


setUp1 <- createBayesianSetup(likelihood1, 
                              lower = c(-500,-500,-500,0.01), 
                              upper = c(500,500,500,3000))
setUp2 <- createBayesianSetup(likelihood2, 
                              lower = c(-500,-500,0.01), 
                              upper = c(500,500,3000))

out1 <- runMCMC(bayesianSetup = setUp1)
M1 = marginalLikelihood(out1, start = 1000)

out2 <- runMCMC(bayesianSetup = setUp2)
M2 = marginalLikelihood(out2, start = 1000)


### Calculating Bayes factor

exp(M1$ln.ML - M2$ln.ML)

# it has therefore been suggested that ML should not be calculated on uninformative priors. But
# what to do if there are no informative priors?
# one option is to calculate the fractional BF, which means that one splites the data in half,
# uses the first half to fit the model, and then use the posterior as a new (now informative)
# prior for the ML - let's do this for the previous case


# likelihoods with half the data 
likelihood1 <- function(param){
  pred = param[1] + param[2]*x + param[3] * x^2
  singlelikelihoods = dnorm(y, mean = pred, sd = 1/(param[4]^2), log = TRUE)
  return(sum(singlelikelihoods[seq(1, 30, 2)]))  
}
likelihood2 <- function(param){
  pred = param[1] + param[2]*x 
  singlelikelihoods = dnorm(y, mean = pred, sd = 1/(param[3]^2), log = TRUE)
  return(sum(singlelikelihoods[seq(1, 30, 2)]))  
}

setUp1 <- createBayesianSetup(likelihood1, 
                              lower = c(-500,-500,-500,0.01), 
                              upper = c(500,500,500,3000))
setUp2 <- createBayesianSetup(likelihood2, 
                              lower = c(-500,-500,0.01), 
                              upper = c(500,500,3000))

out1 <- runMCMC(bayesianSetup = setUp1)
out2 <- runMCMC(bayesianSetup = setUp2)

newPrior1 = createPriorDensity(out1, start = 200, 
                               lower = c(-500,-500,-500,0.01), 
                               upper = c(500,500,500,3000))
newPrior2 = createPriorDensity(out2, start = 200, 
                               lower = c(-500,-500,0.01), 
                               upper = c(500,500,3000))

# now rerun this with likelihoods for the other half of the data and new prior

likelihood1 <- function(param){
  pred = param[1] + param[2]*x + param[3] * x^2
  singlelikelihoods = dnorm(y, mean = pred, sd = 1/(param[4]^2), log = TRUE)
  return(sum(singlelikelihoods[seq(2, 30, 2)]))  
}
likelihood2 <- function(param){
  pred = param[1] + param[2]*x 
  singlelikelihoods = dnorm(y, mean = pred, sd = 1/(param[3]^2), log = TRUE)
  return(sum(singlelikelihoods[seq(2, 30, 2)]))  
}

setUp1 <- createBayesianSetup(likelihood1, prior = newPrior1)
setUp2 <- createBayesianSetup(likelihood2, prior = newPrior2)

out1 <- runMCMC(bayesianSetup = setUp1)
M1 = marginalLikelihood(out1, start = 1000)

out2 <- runMCMC(bayesianSetup = setUp2)
M2 = marginalLikelihood(out2, start = 1000)

### Calculating the fractional Bayes factor

exp(M1$ln.ML - M2$ln.ML)


## End(Not run)

############################################################
### Performance comparison ### 


# Low dimensional case with narrow priors - all methods have low error

# we use a truncated normal for the likelihood to make sure that the density 
# integrates to 1 - makes it easier to calcuate the theoretical ML
likelihood <- function(x) sum(msm::dtnorm(x, log = TRUE, lower = -1, upper = 1))
prior = createUniformPrior(lower = rep(-1,2), upper = rep(1,2))
bayesianSetup <- createBayesianSetup(likelihood = likelihood, prior = prior)
out = runMCMC(bayesianSetup = bayesianSetup, settings = list(iterations = 5000))

# plot(out)

# theoretical value
theory = log(1/(2^2))

marginalLikelihood(out)$ln.ML - theory
marginalLikelihood(out, method = "Prior", numSamples =  500)$ln.ML - theory
marginalLikelihood(out, method = "HM", numSamples =  500)$ln.ML - theory
marginalLikelihood(out, method = "Bridge", numSamples =  500)$ln.ML - theory


# higher dimensions - wide prior - HM and Prior don't work

likelihood <- function(x) sum(msm::dtnorm(x, log = TRUE, lower = -10, upper = 10))
prior = createUniformPrior(lower = rep(-10,3), upper = rep(10,3))
bayesianSetup <- createBayesianSetup(likelihood = likelihood, prior = prior)
out = runMCMC(bayesianSetup = bayesianSetup, settings = list(iterations = 5000))

# plot(out)

# theoretical value
theory = log(1/(20^3))

marginalLikelihood(out)$ln.ML - theory
marginalLikelihood(out, method = "Prior", numSamples =  500)$ln.ML - theory
marginalLikelihood(out, method = "HM", numSamples =  500)$ln.ML - theory
marginalLikelihood(out, method = "Bridge", numSamples =  500)$ln.ML - theory




[Package BayesianTools version 0.1.7 Index]