# Bayes Factors and Marginal Likelihood¶

```
In [2]:
```

```
%matplotlib inline
import pymc3 as pm
import numpy as np
from matplotlib import pyplot as plt
from scipy.special import betaln
from scipy.stats import beta
plt.style.use('seaborn-darkgrid')
print('Running on PyMC3 v{}'.format(pm.__version__))
```

```
Running on PyMC3 v3.4.1
```

The “Bayesian way” to compare models is to compute the *marginal
likelihood* of each model \(p(y \mid M_k)\), *i.e.* the probability
of the observed data \(y\) given the \(M_k\) model. This
quantity, the marginal likelihood, is just the normalizing constant of
Bayes’ theorem. We can see this if we write Bayes’ theorem and make
explicit the fact that all inferences are model-dependant.

where:

- \(y\) is the data
- \(\theta\) the parameters
- \(M_k\) one model out of K competing models

Usually when doing inference we do not need to compute this normalizing constant, so in practice we often compute the posterior up to a constant factor, that is:

However, for model comparison and model averaging the marginal likelihood is an important quantity. Although, it’s not the only way to perform these tasks, you can read about model averaging and model selection using alternative methods here, there and elsewhere.

## Bayesian model selection¶

If our main objective is to choose only one model, the *best* one, from
a set of models we can just choose the one with the largest
\(p(y \mid M_k)\). This is totally fine if all models are assumed to
have the same *a priori* probability. Otherwise, we have to take into
account that not all models are equally likely *a priori* and compute:

Sometimes the main objective is not to just keep a single model but instead to compare models to determine which ones are more likely and by how much. This can be achieved using Bayes factors:

that is, the ratio between the marginal likelihood of two models. The
larger the BF the *better* the model in the numerator (\(M_0\) in
this example). To ease the interpretation of BFs some authors have
proposed tables with levels of *support* or *strength*, just a way to
put numbers into words.

- 1-3: anecdotal
- 3-10: moderate
- 10-30: strong
- 30-100: very strong
- \(>\) 100: extreme

Notice that if you get numbers below 1 then the support is for the model in the denominator, tables for those cases are also available. Of course, you can also just take the inverse of the values in the above table or take the inverse of the BF value and you will be OK.

Is very important to remember that these rules are just conventions, simple guides at best. Results should always be put into context of our problems and should be accompanied with enough details so others could evaluate by themselves if they agree with our conclusions. The evidence necessary to make a claim is not the same in particle physics, or a court, or to evacuate a town to prevent hundreds of deaths.

## Bayesian model averaging¶

Instead of choosing one single model from a set of candidate models, model averaging is about getting one meta-model by averaging the separate models. The Bayesian version of this weights each model by its marginal posterior probability.

This is the optimal way to average models if the prior is *correct* and
the *correct* model is one of the \(M_k\) models in our set.
Otherwise, *bayesian model averaging* will asymptotically select the one
single model in the set of compared models that is closest in
Kullback-Leibler
divergence.

Check this example as an alternative way to perform model averaging.

## Some remarks¶

Now we will briefly discuss some key facts about the *marginal
likelihood*

- The good
**Occam Razor included**: Models with more parameters have a larger penalization than models with fewer parameters. The intuitive reason is that the larger the number of parameters the more*spread*the*prior*with respect to the likelihood.

- The bad
- Computing the marginal likelihood is, generally, a hard task because it’s an integral of a highly variable function over a high dimensional parameter space. In general this integral needs to be solved numerically using more or less sophisticated methods.

- The ugly
- The marginal likelihood depends
**sensitively**on the specified prior \(p(\theta_k \mid M_k)\) for each model.

- The marginal likelihood depends

Notice that *the good* and *the ugly* are related. Using the marginal
likelihood to compare models is a good idea because a penalization for
complex models is already included (thus preventing us from overfitting)
and, at the same time, a change in the prior will affect the
computations of the *marginal likelihood*. At first this sounds a little
bit silly we already know that priors affect computations (otherwise we
could simply avoid them), but the point here is the word
**sensitively**. We are talking about changes in the prior that will
keep inference of \(\theta\) more or less the same, but could have a
big impact in the value of the marginal likelihood.

## Computing Bayes factors¶

The *marginal likelihood* is generally not available in closed-form
except for some restricted models. For this reason many methods have
been devised to compute the *marginal likelihood* and the derived Bayes
factors, some of these methods are so simple and
naive
that works very bad in practice. Most of the useful methods have been
originally proposed in the field of Statistical Mechanics. This
connection is explained because the marginal likelihood is analogous to
a central quantity in statistical physics known as the *partition
function* which in turn is closely related to another very important
quantity the *free-energy*. Many of the connections between Statistical
Mechanics and Bayesian inference are summarized
here.

### Using a hierarchical model¶

Computation of Bayes factors can be framed as a hierarchical model,
where the high-level parameter is an index assigned to each model and
sampled from a categorical distribution. In other words, we perform
inference for two (or more) competing models at the same time and we use
a discrete *dummy* variable that *jumps* between models. How much time
we spend sampling each model is proportional to \(p(M_k \mid y)\).

Some common problems when computing Bayes factors this way is that if one model is better than the other, by definition, we will spend more time sampling from it than from the other model. And this could lead to inaccuracies because we will be undersampling the less likely model. Another problem is that the values of the parameters get updated even when the parameters are not used to fit that model. That is, when model 0 is chosen, parameters in model 1 are updated but since they are not used to explain the data, they only get restricted by the prior. If the prior is too vague, it is possible that when we choose model 1, the parameter values are too far away from the previous accepted values and hence the step is rejected. Therefore we end up having a problem with sampling.

In case we find these problems, we can try to improve sampling by implementing two modifications to our model:

- Ideally, we can get a better sampling of both models if they are visited equally, so we can adjust the prior for each model in such a way to favour the less favourable model and disfavour the most favourable one. This will not affect the computation of the Bayes factor because we have to include the priors in the computation.
- Use pseudo priors, as suggested by Kruschke and others. The idea is simple: if the problem is that the parameters drift away unrestricted, when the model they belong to is not selected, then one solution is to try to restrict them artificially, but only when not used! You can find an example of using pseudo priors in a model used by Kruschke in his book and ported to Python/PyMC3.

If you want to learn more about this approach to the computation of the marginal likelihood see Chapter 12 of Doing Bayesian Data Analysis. This chapter also discuss how to use Bayes Factors as a Bayesian alternative to classical hypothesis testing.

### Analytically¶

For some models, like the beta-binomial model (AKA the *coin-flipping*
model) we can compute the marginal likelihood analytically. If we write
this model as:

the *marginal likelihood* will be:

where:

- \(B\) is the beta function not to get confused with the \(Beta\) distribution
- \(n\) is the number of trials
- \(h\) is the number of success

Since we only care about the relative value of the *marginal likelihood*
under two different models (for the same data), we can omit the binomial
coefficient \(\binom {n}{h}\), thus we can write:

This have been coded in the following cell, with a twist we will be
using the `betaln`

function instead of the `beta`

function, this is
done to prevent underflow.

```
In [6]:
```

```
def beta_binom(prior, y):
"""
Compute the marginal likelihood, analytically, for a beta-binomial model.
prior : tuple
tuple of alpha and beta parameter for the prior (beta distribution)
y : array
array with "1" and "0" corresponding to the success and fails respectively
"""
alpha, beta = prior
h = np.sum(y)
n = len(y)
p_y = np.exp(betaln(alpha + h, beta+n-h) - betaln(alpha, beta))
return p_y
```

Our data for this example consist on 100 “flips of a coin” and the same
number of observed “heads” and “tails”. We will compare two models one
with a uniform prior and one with a *more concentrated* prior around
\(\theta = 0.5\)

```
In [7]:
```

```
y = np.repeat([1, 0], [50, 50]) # 50 "heads" and 50 "tails"
priors = ((1, 1), (30, 30))
```

```
In [8]:
```

```
for a, b in priors:
distri = beta(a, b)
x = np.linspace(0, 1, 100)
x_pdf = distri.pdf(x)
plt.plot (x, x_pdf, label=r'$\alpha$ = {:d}, $\beta$ = {:d}'.format(a, b))
plt.yticks([])
plt.xlabel('$\\theta$')
plt.legend()
```

The following cell returns the Bayes factor

```
In [9]:
```

```
BF = (beta_binom(priors[1], y) / beta_binom(priors[0], y))
print(round(BF))
```

```
5.0
```

We see that the model with the more concentrated prior \(Beta(30, 30)\) has \(\approx 5\) times more support than the model with the more extended prior \(Beta(1, 1)\). Besides the exact numerical value this should not be surprising since the prior for the most favoured model is concentrated around \(\theta = 0.5\) and the data \(y\) has equal number of head and tails, consintent with a value of \(\theta\) around 0.5.

## Bayes factors and inference¶

In this example we have used Bayes factors to judge which model seems to
be better at explaining the data, and we get that one of the models is
\(\approx 5\) *better* than the other.

But what about the posterior we get from these models? How different they are?

```
In [12]:
```

```
pm.summary(traces[0], varnames='a').round(2)
```

```
Out[12]:
```

mean | sd | mc_error | hpd_2.5 | hpd_97.5 | |
---|---|---|---|---|---|

a | 0.5 | 0.05 | 0.0 | 0.41 | 0.6 |

```
In [13]:
```

```
pm.summary(traces[1], varnames='a').round(2)
```

```
Out[13]:
```

mean | sd | mc_error | hpd_2.5 | hpd_97.5 | |
---|---|---|---|---|---|

a | 0.5 | 0.03 | 0.0 | 0.44 | 0.57 |

We may argue that the results are pretty similar, we have the same mean
value for \(\theta\), and a slightly wider posterior for
`model_0`

, as expected since this model has a wider prior. We can also
check the posterior predictive distribution to see how similar they are.

```
In [22]:
```

```
_, ax = plt.subplots()
ppc_0 = pm.sample_ppc(traces[0], 1000, models[0], size=(len(y), 20))
ppc_1 = pm.sample_ppc(traces[1], 1000, models[1], size=(len(y), 20))
for m_0, m_1 in zip(ppc_0['yl'].T, ppc_1['yl'].T):
pm.kdeplot(np.mean(m_0, 0), ax=ax, color='C0', label='model 0')
pm.kdeplot(np.mean(m_1, 0), ax=ax, color='C1', label='model 1')
plt.xlabel(u'θ') # matplotlib might have issues with displaying 'theta' string
plt.yticks([]);
```

```
100%|██████████| 1000/1000 [00:02<00:00, 490.58it/s]
100%|██████████| 1000/1000 [00:01<00:00, 554.74it/s]
```

Summarizing, while is true that the observed data \(y\) is more
consistent with `model_1`

than `model_0`

, the posteriors
distributions we got from both models are similar and hence if we use
these posteriors to make predictions we will get similar results.