# Introduction

This is meant to be shorter blog post than normal with the expectation that the material will be developed further in future blog posts.

A Bayesian will have a prior view of the distribution of some data and then based on data, update that view. Mostly the updated distribution, the posterior, will not be expressible as an analytic function and sampling via Markov Chain Monte Carlo (MCMC) is the only way to determine it.

In some special cases, when the posterior is of the same family of distributions as the prior, then the posterior is available analytically and we call the posterior and prior conjugate. It turns out that the normal or Gaussian distribution is conjugate with respect to a normal likelihood distribution.

This gives us the opportunity to compare MCMC against the analytic solution and give ourselves more confidence that MCMC really does deliver the goods.

Some points of note:

• Since we want to display the posterior (and the prior for that matter), for histograms we use the histogram-fill package.

• Since we are using Monte Carlo we can use all the cores on our computer via one of Haskell’s parallelization mechanisms.

## Preamble

> {-# OPTIONS_GHC -Wall                      #-}
> {-# OPTIONS_GHC -fno-warn-type-defaults    #-}
> {-# OPTIONS_GHC -fno-warn-unused-do-bind   #-}
> {-# OPTIONS_GHC -fno-warn-missing-methods  #-}
> {-# OPTIONS_GHC -fno-warn-orphans          #-}

> {-# LANGUAGE NoMonomorphismRestriction     #-}

> module ConjMCMCSimple where
>
> import qualified Data.Vector.Unboxed as V
> import Data.Random.Source.PureMT
> import Data.Random
> import Data.Histogram ( asList )
> import qualified Data.Histogram as H
> import Data.Histogram.Fill
> import Data.Histogram.Generic ( Histogram )
> import Data.List
> import Control.Parallel.Strategies
>
> import Diagrams.Backend.Cairo.CmdLine
>
> import Diagrams.Backend.CmdLine
> import Diagrams.Prelude hiding ( sample, render )
>
> import LinRegAux


# A Simple Example

## Analytically

Suppose the prior is $\mu \sim \cal{N}(\mu_0, \sigma_0)$, that is

$\displaystyle \pi(\mu) \propto \exp{\bigg( -\frac{(\mu - \mu_0)^2}{2\sigma_0^2}\bigg)}$

Our data is IID normal, $x_i \sim \cal{N}(\mu, \sigma)$, where $\sigma$ is known, so the likelihood is

$\displaystyle p(x\,|\,\mu, \sigma) \propto \prod_{i=1}^n \exp{\bigg( -\frac{(x_i - \mu)^2}{2\sigma^2}\bigg)}$

The assumption that $\sigma$ is known is unlikely but the point of this post is to demonstrate MCMC matching an analytic formula.

This gives a posterior of

\displaystyle \begin{aligned} p(\mu\,|\, \boldsymbol{x}) &\propto \exp{\bigg( -\frac{(\mu - \mu_0)^2}{2\sigma_0^2} - \frac{\sum_{i=1}^n(x_i - \mu)^2}{2\sigma^2}\bigg)} \\ &\propto \exp{\bigg[-\frac{1}{2}\bigg(\frac{\mu^2 \sigma^2 -2\sigma^2\mu\mu_0 - 2\sigma_0^2n\bar{x}\mu + \sigma_0^2 n\mu^2}{\sigma^2\sigma_0^2}\bigg)\bigg]} \\ &= \exp{\bigg[-\frac{1}{2}\bigg(\frac{ (n\sigma_0^2 + \sigma^2)\mu^2 - 2(\sigma^2\mu_0 - \sigma_0^2n\bar{x})\mu}{\sigma^2\sigma_0^2}\bigg)\bigg]} \\ &= \exp{\Bigg[-\frac{1}{2}\Bigg(\frac{ \mu^2 - 2\mu\frac{(\sigma^2\mu_0 - \sigma_0^2n\bar{x})}{(n\sigma_0^2 + \sigma^2)}}{\frac{\sigma^2\sigma_0^2}{(n\sigma_0^2 + \sigma^2)}}\Bigg)\Bigg]} \\ &\propto \exp{\Bigg[-\frac{1}{2}\Bigg(\frac{\big(\mu - \frac{(\sigma^2\mu_0 - \sigma_0^2n\bar{x})}{(n\sigma_0^2 + \sigma^2)}\big)^2}{\frac{\sigma^2\sigma_0^2}{(n\sigma_0^2 + \sigma^2)}}\Bigg)\Bigg]} \end{aligned}

In other words

$\displaystyle \mu\,|\, \boldsymbol{x} \sim {\cal{N}}\bigg(\frac{\sigma^2\mu_0 + n\sigma_0^2\bar{x}}{n\sigma_0^2 + \sigma^2}, \frac{\sigma^2\sigma_0^2}{n\sigma_0^2 + \sigma^2} \bigg)$

Writing

$\displaystyle \sigma_n^2 = \frac{\sigma^2\sigma_0^2}{n\sigma_n^2 + \sigma^2}$

we get

$\displaystyle \frac{1}{\sigma_n^2} = \frac{n}{\sigma^2} + \frac{1}{\sigma_0^2}$

Thus the precision (the inverse of the variance) of the posterior is the precision of the prior plus the precision of the data scaled by the number of observations. This gives a nice illustration of how Bayesian statistics improves our beliefs.

Writing

$\displaystyle \mu_n = \frac{\sigma^2\mu_0 + n\sigma_0^2\bar{x}}{n\sigma_0^2 + \sigma^2}$

and

$\displaystyle \lambda = 1 / \sigma^2, \, \lambda_0 = 1 / \sigma_0^2, \, \lambda_n = 1 / \sigma_n^2$

we see that

$\displaystyle \mu_n = \frac{n\bar{x}\lambda + \mu_0\lambda_0}{\lambda_n}$

Thus the mean of the posterior is a weight sum of the mean of the prior and the sample mean scaled by preciscion of the prior and the precision of the data itself scaled by the number of observations.

Rather arbitrarily let us pick a prior mean of

> mu0 :: Double
> mu0 = 11.0


and express our uncertainty about it with a largish prior variance

> sigma_0 :: Double
> sigma_0 = 2.0


And also arbitrarily let us pick the know variance for the samples as

> sigma :: Double
> sigma = 1.0


We can sample from this in way that looks very similar to STAN and JAGS:

> hierarchicalSample :: MonadRandom m => m Double
> hierarchicalSample = do
>   mu <- sample (Normal mu0 sigma_0)
>   x  <- sample (Normal mu sigma)
>   return x


and we didn’t need to write a new language for this.

Again arbitrarily let us take

> nSamples :: Int
> nSamples = 10


and use

> arbSeed :: Int
> arbSeed = 2


And then actually generate the samples.

> simpleXs :: [Double]
> simpleXs =
>   evalState (replicateM nSamples hierarchicalSample)
>             (pureMT $fromIntegral arbSeed)  Using the formulae we did above we can calculate the posterior > mu_1, sigma1, simpleNumerator :: Double > simpleNumerator = fromIntegral nSamples * sigma_0**2 + sigma**2 > mu_1 = (sigma**2 * mu0 + sigma_0**2 * sum simpleXs) / simpleNumerator > sigma1 = sigma**2 * sigma_0**2 / simpleNumerator  and then compare it against the prior The red posterior shows we are a lot more certain now we have some evidence. ## Via Markov Chain Monte Carlo The theory behinde MCMC is described in a previous post. We need to generate some proposed steps for the chain. We sample from the normal distribution but we could have used e.g. the gamma. > normalisedProposals :: Int -> Double -> Int -> [Double] > normalisedProposals seed sigma nIters = > evalState (replicateM nIters (sample (Normal 0.0 sigma))) > (pureMT$ fromIntegral seed)


We also need samples from the uniform distribution

> acceptOrRejects :: Int -> Int -> [Double]
> acceptOrRejects seed nIters =
>   evalState (replicateM nIters (sample stdUniform))
>   (pureMT $fromIntegral seed)  And now we can calculate the (un-normalised) prior, likelihood and posterior > prior :: Double -> Double > prior mu = exp (-(mu - mu0)**2 / (2 * sigma_0**2)) > > likelihood :: Double -> [Double] -> Double > likelihood mu xs = exp (-sum (map (\x -> (x - mu)**2 / (2 * sigma**2)) xs)) > > posterior :: Double -> [Double] -> Double > posterior mu xs = likelihood mu xs * prior mu  The Metropolis algorithm tells us that we always jump to a better place but only sometimes jump to a worse place. We count the number of acceptances as we go. > acceptanceProb :: Double -> Double -> [Double] -> Double > acceptanceProb mu mu' xs = min 1.0 ((posterior mu' xs) / (posterior mu xs))  > oneStep :: (Double, Int) -> (Double, Double) -> (Double, Int) > oneStep (mu, nAccs) (proposedJump, acceptOrReject) = > if acceptOrReject < acceptanceProb mu (mu + proposedJump) simpleXs > then (mu + proposedJump, nAccs + 1) > else (mu, nAccs)  Now we can actually run our simulation. We set the number of jumps and a burn in but do not do any thinning. > nIters, burnIn :: Int > nIters = 300000 > burnIn = nIters div 10  Let us start our chain at > startMu :: Double > startMu = 10.0  and set the variance of the jumps to > jumpVar :: Double > jumpVar = 0.4  > test :: Int -> [(Double, Int)] > test seed = > drop burnIn$
>   scanl oneStep (startMu, 0) $> zip (normalisedProposals seed jumpVar nIters) > (acceptOrRejects seed nIters)  We put the data into a histogram > numBins :: Int > numBins = 400  > hb :: HBuilder Double (Data.Histogram.Generic.Histogram V.Vector BinD Double) > hb = forceDouble -<< mkSimple (binD lower numBins upper) > where > lower = startMu - 1.5*sigma_0 > upper = startMu + 1.5*sigma_0 > > hist :: Int -> Histogram V.Vector BinD Double > hist seed = fillBuilder hb (map fst$ test seed)


Not bad but a bit lumpy. Let’s try a few runs and see if we can smooth things out.

> hists :: [Histogram V.Vector BinD Double]
> hists = parMap rpar hist [3,4..102]

> emptyHist :: Histogram V.Vector BinD Double
> emptyHist = fillBuilder hb (replicate numBins 0)
>
> smoothHist :: Histogram V.Vector BinD Double
> smoothHist = foldl' (H.zip (+)) emptyHist hists


Quite nice and had my machine running at 750% with +RTS -N8.

## Comparison

Let’s create the same histogram but from the posterior created analytically.

> analPosterior :: [Double]
> analPosterior =
>   evalState (replicateM 100000 (sample (Normal mu_1 (sqrt sigma1))))
>   (pureMT $fromIntegral 5) > > histAnal :: Histogram V.Vector BinD Double > histAnal = fillBuilder hb analPosterior  And then compare them. Because they overlap so well, we show the MCMC, both and the analytic on separate charts. # PostAmble Normally with BlogLiteratelyD, we can generate diagrams on the fly. However, here we want to run the simulations in parallel so we need to actually compile something. ghc -O2 ConjMCMCSimple.lhs -main-is ConjMCMCSimple -threaded -fforce-recomp > displayHeader :: FilePath -> Diagram B R2 -> IO () > displayHeader fn = > mainRender ( DiagramOpts (Just 900) (Just 700) fn > , DiagramLoopOpts False Nothing 0 > )  > main :: IO () > main = do > displayHeader "http://idontgetoutmuch.files.wordpress.com/2014/03/HistMCMC.png" > (barDiag MCMC > (zip (map fst$ asList (hist 2)) (map snd $asList (hist 2))) > (zip (map fst$ asList histAnal) (map snd $asList histAnal))) > > displayHeader "http://idontgetoutmuch.files.wordpress.com/2014/03/HistMCMCAnal.png" > (barDiag MCMCAnal > (zip (map fst$ asList (hist 2)) (map snd $asList (hist 2))) > (zip (map fst$ asList histAnal) (map snd $asList histAnal))) > > displayHeader "http://idontgetoutmuch.files.wordpress.com/2014/03/HistAnal.png" > (barDiag Anal > (zip (map fst$ asList (hist 2)) (map snd $asList (hist 2))) > (zip (map fst$ asList histAnal) (map snd $asList histAnal))) > > displayHeader "http://idontgetoutmuch.files.wordpress.com/2014/03/SmoothHistMCMC.png" > (barDiag MCMC > (zip (map fst$ asList smoothHist) (map snd $asList smoothHist)) > (zip (map fst$ asList histAnal) (map snd \$ asList histAnal)))