Simple model selection example in PYMC

Question

I am currently experimenting with PYMC and I am trying out a simple example so that I start learning how things work (I am also a Python beginner, but an experienced machine learner).

I have set myself the following very simple model selection task:

Imagine we have a dataset that has been entirely generated either from model 0 or model 1 (not a mixture).

• Model 0 is a normal distribution with mean m0 and standard deviation s0.

• Likewise, model 1 is also a normal distribution with mean m1 and standard deviation s1.

• We assume that we know all parameters m0,s0,m1,s1.

• We postulate a hidden binary random variable V which indicates the model that really generated the dataset.

What we would like to do is infer the posterior of V.

I have made an attempt in the code below, but unfortunately after trying for a couple of days, I capitulate and throw myself at your feet for assistance.

The example is of course very simple and may appear contrived. What I would like to do in the future, is use PYMC for model selection. Hence, how one correctly handles a hidden indicator variable like V is important to me. Unfortunately, in my code I keep getting weird results for the posterior of V.

I hope the code below makes sense. I thank you all for your time.

from pymc  import *
import matplotlib 
from pylab import hist, show, plot, title, figure
import numpy as np
from numpy import *


#-----------------------------------------------
# Generate some data
#-----------------------------------------------

# Define means for gaussians
mu0 =  5.0
mu1 = -5.0

# Define standard deviations for gaussians 
s0 = 1.0
s1 = 1.0


# This variable chooses from which model we generate the observed data.
# It can be either 0 or 1
true_model = 0

# number of data items
numData = 100

if true_model==0:
    # Generate data from first model
    data = np.random.randn(numData,1)*s0 + mu0
elif true_model==1:
    # Generate data from second model
    data = np.random.randn(numData,1)*s1 + mu1


#-----------------------------------------------
# Define variables
#-----------------------------------------------
PR = Dirichlet("priorOnModels", theta=(0.5,0.5)) # both models are equally likely
V  = Categorical('V', p=PR)


#-----------------------------------------------
# Define model log-likelihood
#-----------------------------------------------
@potential
def mymodel_loglikel(sw=V):

    if sw==0:        
        loglikel = distributions.normal_like(data, mu0, 1.0/s0**2)

    elif sw==1:        
        loglikel = distributions.normal_like(data, mu1, 1.0/s1**2)

    return loglikel


#-----------------------------------------------
# Run sampler
#-----------------------------------------------
simplemodel = Model([mymodel_loglikel, PR, V])
mc = MCMC(simplemodel)
mc.sample(iter=12000,burn=2000)

figure()
hist(data)
Matplot.plot(mc)
print "expectation of V variable is %.3f" % np.mean(mc.trace('V')[:])

show()

EDIT 2: Updated after Tom's suggestions

In response to a comment below, the weird behaviour I get concerning V, is that its mean posterior is always 0.0. Judging from the trace, it seems that V is not properly sampled. I suspect a minor programming error somewhere, but can't pinpoint it.

I attach below a figure of the trace plots of V.

Answer 1

When defining a potential, the function should return the log-probability. So instead of

return np.exp(loglikel)

You want

return loglikel

Another thing to watch out for is the expression 1/s0**2. Make sure that s0 is float in this expression, or change it to 1.0/s0**2.

PyMC also has a bug in its choice of proposals for binary variables (documented in section 5.7.3), which I first noticed in another answer. The fix is to add the line:

mc.use_step_method(DiscreteMetropolis, V, proposal_distribution='Prior')