Markov chain Monte Carlo with PyMC
Markov Chain Monte Carlo (MCMC) is a technique for generating a sample from a distribution, and it works even if all you have is a non-normalized representation of the distribution. Why does a data scientist care about this? Well, in a Bayesian analysis a non-normalized form of the posterior distribution is super easy to come by, being just the product of likelihood and prior - so MCMC can be used to sample from (essentially simulate) a Bayesian posterior. In python one of the most widely used packages for doing exactly this is called PyMC.
What is Markov chain Monte Carlo
A Markov Chain is a sequence of RVs {X} each of which will have an observed value from the state space of possible values, {x}. A Markov Process is a sequence of such RVs where the distribution of the initial RV's value is specified (Π0) as well as a Transition Rule (P) which gives the probability to transition from one state to another, P(i,j)=P(Xn+1=xj|Xn=xi), for all pairs of states.
Notice P does not care what happened in the chain leading up to the current state, this "memorylessness" is an important element of the definition of a Markov Process. Time-reversible Markov Processes are those which satisfy μiP(i,j)=μjP(j,i) for some distribution μ. Such processes have the guarantee that the long-run fractions of state values in the sequence approaches μ. Thus if we want a sample from a distribution μ we could just find a P with which it satisfies time-reversibility and then simulate a long Markov Chain.
The Metropolis-Hastings Method is an algorithm for constructing such a P given only a non-normalized form of the μ of interest. So given a non-normalized posterior from a Bayesian analysis, we can run an MCMC and get a simulated sample from it, which allows us to estimate various things about this posterior distribution.
Now that all of that is 100% super clear and obvious, lets get our environment set up:
from matplotlib import pyplot as plt import pymc import numpy as np log = np.log pi = np.pi
PyMC: Markov Chain Monte Carlo in Python
PyMC is a python package that helps users define stochastic models and then construct Bayesian posterior samples via MCMC. There are two main object types which are building blocks for defining models in PyMC: Stochastic and Deterministic variables. All PyMC models are linked groups of such variable objects, meaning linked together by being passed as inputs to one another. As discussed below, a potential point of confusion is the Deterministic class is actually used to instantiate random variables (which are definitively non-deterministic).
Stochastic and Deterministic Subclasses (Variables)
All Stochastic and Deterministic variables in PyMC have parents (variables that are their inputs) and children (variables which take them as an input). Variables represented by the Deterministic subclass are those whose specific value is completely determined by the value of their parents, while variables represented by the Stochastic subclass have a probability distribution for their value given the values of the parents. This is confusing terminology because often Deterministic-implemented variables are RVs since their parents are RVs.
To prepare for implementing your system in PyMC you should write down your model in the following form:
- distribution of the data given all the relevant quantities
- deterministic expressions for any deterministic quantities
- distributions for any uncertain quantities
Setting Up a PyMC Model
Here is an excellent example: a data set of the number of coal mine disasters per year over many years, defined as an numpy array below:
disasters_array = np.array([ 4, 5, 4, 0, 1, 4, 3, 4, 0, 6, 3, 3, 4, 0, 2, 6,
3, 3, 5, 4, 5, 3, 1, 4, 4, 1, 5, 5, 3, 4, 2, 5,
2, 2, 3, 4, 2, 1, 3, 2, 2, 1, 1, 1, 1, 3, 0, 0,
1, 0, 1, 1, 0, 0, 3, 1, 0, 3, 2, 2, 0, 1, 1, 1,
0, 1, 0, 1, 0, 0, 0, 2, 1, 0, 0, 0, 1, 1, 0, 2,
3, 3, 1, 1, 2, 1, 1, 1, 1, 2, 4, 2, 0, 0, 1, 4,
0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0, 1])
The disasters are going to be modeled as Poisson events with the goal of identifying which year a change in the rate parameter occurs (due to regulation being passed). Lets define some quantities for this system so that we can set up a model:
- D(t) is the data as a function of year
- r(t) is the rate parameter which also dependson the year since there is a point at which it switches
- s is the year of the switch
- e is the earlier value of s while l is the later value of s
- tl an th are the first and last years of the data set.
Here would be a good way to write out this model before even touching your keyboard: (D(t)|e,s,l)∼Poisson(r(t))r(t)={e lt≤s, t≥s; s∼DiscreteUniform(tl,th) e∼Exponential(re) l∼Exponential(rl)
If you can't write down something like the above on pencil and paper then you aren't ready to code and you'll want to spend some more time mathematically representing your system and specifying your model. Assuming you're good with all that, then you need to start out by implementing each variable as an appropriate object in PyMC. In this case the only thing we would implement as Deterministic would be r(t), whose parents s, l and e fully determine it. The Stochastic implementation of s, e, and l is our Bayesian prior (re and rl are just specific constants we need to choose for these priors), while the stochastic implementation of D(t) is our likelihood.
PyMC defines subclasses of Stochastic for lots of common distributions, so that we can quickly and easily instantiate the RVs in our model. The functions that construct these stochastic objects take the "handle" of the stochastic quantity as their first input. Let's try it out by specifying our priors:
s = pymc.DiscreteUniform("s", lower=0, upper=110) # Returns an object of the stochastic subclass discrete uniform
e = pymc.Exponential("e", beta=1) # Choose r_e to be 1 gives a reasonable prior
l = pymc.Exponential("l", beta=1) # Choose r_l to be 1 gives a reasonable prior
In addition, for each common distribution PyMC defines a function suffixed by _like() which returns the log-likelihood of a value under that distribution, e.g.
logprob = discrete_uniform_like(x=0.5, lower=0, upper=110)
Remember that we have one variable that is deterministic - r. To instantiate RVs that are represented by Determinisic class we need to use a decorator defined in the module @deterministic. The decorator function should return the value of the variable for the given input (parent) values:
@pymc.deterministic(plot=False)
def r(s=s, e=e, l=l):
"""Concatenate the two Poisson mean values"""
out = np.empty(len(disasters_array))
out[:s] = e
out[s:] = l
return out
Notice that s and e are actually Stochastic objects, but within the definition of r we are able to treat them like numbers (as slice indexes). This is because for objects used as parents of a variable, PyMC will replace the objects with their present values whenever the variables value or log-probability needs to be computed.
The data D is a final stochastic random vector, but it is special in that we have a fixed set of observed values for it - this is indicated to PyMC by the observed keyword:
D = pymc.Poisson("D", mu=r, value=disasters_array, observed=True)
As an aside, you can also create Stochastic objects by decorating functions (@stochastic) that compute their log-probabilities (whereas for @deterministic decorates functions which compute the variable value).
All the above variables constitute our model and the nature of the linkages amongst them can be examined by looking at the children and parents attribute of each variable. For instance:
D.parents
{'mu': pymc.PyMCObjects.Deterministic 'r' at 0x000001F519677EB8}
This is a dictionary - it tells us that the stochastic variable D has one parent, the Deterministic object called "r", and htis object is labeled as "mu" in the internal workings of D. We expect that r should have D as it's child:
r.children
{pymc.distributions.new_dist_class.locals.new_class 'D' at 0x000001F5196779B0}
A model (collection of linked varibles) can be represented by a directed acyclic graph where e.g. unobserved stochastic variables are open ellipses, observed stochastic variables are filled ellipse, and deterministic variables are triangles. Arrows point from parent to child and display the label that the child assigns to the parent. You can create the model from all of our defined variables, and then print the graph for your model, but it requires an install so I've commented it out below:
minemodel = pymc.Model([e, l, s, r, D]) #pymc.graph.graph(minemodel)
All PyMC variables have an attribute value that stores the current value of that variable (randomly initialized if not observed) and furthermore stochastic objects can evaluate their pmf or pdf at their current value given the current values of their parents. The logp attribute returns this log likelihood (for random vectors like D it returns the sum of the logarithms of the joint probability or density of all elements of the value).
For example, since we specified our prior on s to be discrete uniform on 1 to 110, we expect it to be randomly initialzed somewhere in that range, and the log probability for that value should be log10(1/110):
print(s.parents) print(s.value) print(np.exp(s.logp)) print(1/110) # for good measure
{'upper': 110, 'lower': 0}
41
0.00900900900901
0.00909090909090909
Fitting PyMC Models
PyMC provides several objects that fit probability models (linked collections of variables) - the primary such object being MCMC (fits with Markov chain Monte Carlo algorithm). "Fitting" a Bayesian model means characterizing the posterior distribution somehow. Here are trying to represent the posterior p(s,e,l|D) by a set of joint samples from it - the MCMC sampler randomly updates the values of s, e and l according to the Metropolis-Hastings algorithm. You can create an MCMC fitting object for your model by:
sampler = pymc.MCMC(minemodel)
The fitting object M has attributes which correspond to the variables we put into it e.g. M.D, M.s etc. and methods that let us conduct the fit. For example:
M.isample(number of iterations, burn-in length, thinning interval)
will start a fitting routine that can be interrupted at any time (the "i" means "interactive"). The burn-in should be some number of algorithm steps beyond which you reasonably expect convergence. The thinning parameter retains only every kth draw and mitigates auto-correlation. This is useful when you compute statistics from the posterior and want to assume like your MCMC sample is a set of iid draws from the posterior. For instance, if you use the MCMC sample mean as an estimator for the true posterior mean then you might want to say something about it's distribution - much easier if the sample is iid. If you don't have a good idea for burn or thin then keep everything and inspect it all!
# Will create a sample of 10000 draws from the posterior by Met-Hast algo sampler.sample(iter=10000, burn=1000, thin=10)
[-----------------100%-----------------] 10000 of 10000 complete in 1.7 sec
Inspecting MCMC Results
The result of calling the sample() method is that a new attribute is created in M, the fitting object, which stores the resulting samples - these can be accessed by the trace method of the fitting object:
s_tr = sampler.trace("s")
pymc.database.ram.Trace
sampler.trace("s")[1:10] # Just look at the first few values
%matplotlib inline
plt.hist(s_tr[:],bins=20)
type(sampler.trace("s"))
array([37, 37, 37, 37, 37, 37, 37, 37, 37])
(array([ 2., 3., 0., 4., 0., 5., 71., 186., 34.,
0., 131., 130., 188., 77., 0., 20., 14., 3.,
30., 2.]),
array([ 31. , 31.8, 32.6, 33.4, 34.2, 35. , 35.8, 36.6, 37.4,
38.2, 39. , 39.8, 40.6, 41.4, 42.2, 43. , 43.8, 44.6,
45.4, 46.2, 47. ]),
a list of 20 Patch objects)
The Matplot submodule of PyMC defines a plot(model) function which defaults to a very useful visualization of the trace of a model, plotting the Stochastic components in terms of time series of Metropolis-Hastings and a histogram of values.
pymc.Matplot.plot(sampler)
Calculating Probabilities from MCMC Results
Trace objects have a nice little method called .stats() which returns some good descriptive statistics of the sample:
s_tr.stats()
{'95% HPD interval': array([ 35., 44.]),
'mc error': 0.1183359415466341,
'mean': 39.483333333333334,
'n': 900,
'quantiles': {2.5: 36.0, 25: 37.0, 50: 40.0, 75: 41.0, 97.5: 46.0},
'standard deviation': 2.4972762940629716}
Alternatively the method stats() for a Fitting Object returns a dictionary with keys for each variable and entries with nice summary values.
sampler.stats()["e"]
{'95% HPD interval': array([ 2.53940542, 3.62556813]),
'mc error': 0.0095947658494946655,
'mean': 3.0697409128224913,
'n': 900,
'quantiles': {2.5: 2.5394054181768095,
25: 2.8756569678436681,
50: 3.0595943198980571,
75: 3.2518509997277714,
97.5: 3.6255681335759564},
'standard deviation': 0.28231161815605127}
sampler.stats()["e"]["mean"] # Same as np.mean(M.trace("e")[:])
3.0697409128224913
Summary
MCMC is a surprisingly difficult and bug-prone algorithm to implement by hand, and that's why PyMC makes it much easier and less stressful. PyMC also makes the work more dynamic; getting hand-coded MCMC’s working used to be so much work being reluctant to change anything, but with PyMC changing models is much easier.
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