Bayes Factor

The Bayes Factor of model class M₁ to model class M₂ for some set of observations X is the ratio of their associated marginal class likelihoods. The marginal class likelihood of some set of observations is the marginal likelihood of the observations for the model class obtained by marginalizing the joint probability distribution of the observations and the model class parameters by treating the model class parameters $\theta$ as nuisance parameters, i.e.,

$m_{\theta }(X|M)=\int _{\theta }p(X,\theta |M)\,d\theta =\int _{\theta }p(X|\theta ,M)\,p(\theta |M)\,d\theta$

showing that it may be calculated directly using the joint probability distribution or from the conditional probability distribution where the second form of the integral was obtained using the Bayesian Product Rule. Thus the Bayes factor for model class M₁ with parameters $\theta _{1}$ to model class M₂ with parameters $\theta _{2}$ is

$BF(X,M_{1},M_{2})={\frac {m_{\theta _{1}}(X|M_{1})}{m_{\theta _{2}}(X|M_{2})}}$

In determining which model class has the largest posterior odds ratio ratio (of which the Bayes factor is a principle component) when compared against all others, the model class or mathematical model which best explains the data is determined. It is then left to determine the best inference as to the values of that models parameters via parameter estimation.