# Formal proof of Occam's razor for nested models

I consider 2 models $$M_0$$ and $$M_1$$, $$M_1$$ being more complicated than $$M_0$$ in the sense that it has more parameters (I usually assume than $$M_0$$ is nested within $$M_1$$). They are respectively parametrized by $$\theta_0$$ and $$\theta_1$$. I assume that

1. $$\theta_0 \subset \theta_1$$ (i.e. $$M_1$$ has the same parameters as $$M_0$$ plus extra parameters)
2. $$p(\theta_0|M_1) = p(\theta_0|M_0)$$ (both models have the same priors for the parameters they have in common)

I would like to prove the following inequality:

$$\forall \theta_0 \\ \langle \log p(\mathcal{D | M_0}) \rangle _{p(\mathcal{D | \theta_0, M_0})} \geq \langle \log p(\mathcal{D | M_1}) \rangle _{p(\mathcal{D | \theta_0, M_0})}$$

i.e. that on average, if my data $$\mathcal{D}$$ are generated from $$M_0$$ parametrized with a given $$\theta_0$$, then the Bayes factor is going to favor $$M_0$$ over $$M_1$$.

Has it already been done ? Intuitively, it is an application of Occam's razor (a simpler and true model will be favored over a more complicated one), but I lack a formal proof.

Precision on the notations : $$p(\mathcal{D}|M_0,\theta_0)$$ is not the same as $$p(\mathcal{D}|M_0)$$, and I thus cannot use the positivity of the Kullback-Leibler divergence. In "$$M_0,\theta_0$$", I specify both the model and its parameters. In "$$M_0$$", I only specify the model. $$p(\mathcal{D}|M_0,\theta_0)$$ is the probability that the data $$\mathcal{D}$$ are generated from model $$M_0$$ with parameters $$\theta_0$$, while $$p(\mathcal{D}|M_0)$$ is the marginal likelihood over all parameters (the one we use to compute the Bayes factor) : $$\int_{\theta} p(\mathcal{D}|M_0,\theta)p(\theta|M_0)$$ where $$p(\theta|M_0)$$ is the prior of parameters under model $$M_0$$.

• If I understand your notation correctly, the difference between the two expressions would be the Kullback-Leibler divergence or "relative entropy" between the models (for any fixed values of $\theta_0$ and $\theta_1$). Your inequality appears to be Gibbs' Inequality.
– whuber
Feb 12, 2020 at 13:48
• Sadly, it is not. $p(\mathcal{D}|M_0,\theta_0)$ is not the same as $p(\mathcal{D}|M_0)$, and I thus cannot use the positivity of the Kullback-Leibler divergence. Feb 12, 2020 at 17:47
• Then could you please clarify the difference between "$M_0,\theta_0$" and "$M_0$"?
– whuber
Feb 12, 2020 at 18:31
• This cannot be proven true because there seems to be no guarantees that $p(\theta|M_0)$ won't assign $0$ probability to $\theta_0$, while $p(\theta|M_1)$ assigning non-zero probability to $\theta_0$ Feb 12, 2020 at 19:38
• The result cannot hold in general as it depends on the choice of the priors over both models. As an extreme example. take priors degenerated at $\theta_0$. Feb 13, 2020 at 7:59

Here is my attempt at answering the question:

Proposition: Let $$\mathcal{M}_0$$ and $$\mathcal{M}_1$$ two nested models such that $$\mathcal{M}_0 \preceq \mathcal{M}_1$$. We note $$\Theta_0$$ and $$\Theta_1$$ the space of possible parameters for $$\mathcal{M}_0$$ and $$\mathcal{M}_1$$, with $$\Theta_0 \subset \Theta_1$$. If data generated from $$\mathcal{M}_0$$ and $$\mathcal{M}_1$$ are IID, then the following inequality holds $$\forall \theta_0^* \in \Theta_0$$:

$$\begin{equation} \label{eq:proposition1} \langle \log p(\mathcal{D}|\mathcal{M}_0) \rangle _{p(\mathcal{D}| \theta_0^*,\mathcal{M}_0)} \geq \langle \log p(\mathcal{D}|\mathcal{M}_1) \rangle _{p(\mathcal{D}| \theta_0^*,\mathcal{M}_0)} \end{equation}$$

If data are not IID, a sufficient condition for the inequality to hold is

$$\begin{equation} \label{eq:condition1} k_{\mathcal{M}_0} \log (2 \pi) - \sum_{i=1}^{k_{\mathcal{M}_0}} \langle \log (\lambda_{i}^0) \rangle _{p(\mathcal{D}| \theta_0^*,\mathcal{M}_0)} \geq k_{\mathcal{M}_1} \log (2 \pi) - \sum_{i=1}^{k_{\mathcal{M}_1}} \langle \log (\lambda_{i}^1) \rangle _{p(\mathcal{D}| \theta_0^*,\mathcal{M}_0)} \end{equation}$$

where

$$k_{\mathcal{M}_0}$$ and $$k_{\mathcal{M}_1}$$ are the number of independent parameters of $$\mathcal{M}_0$$ and $$\mathcal{M}_1$$;

$$H_0(\hat{\theta}_0)$$ and $$H_1(\hat{\theta}_1)$$ are the Hessian matrices of the log-likelihoods $$p(\mathcal{D}|\theta_0,\mathcal{M}_0)$$ and $$p(\mathcal{D}|\theta_1,\mathcal{M}_1)$$ expressed at their respective MLEs;

$$\{\lambda^0_i\}_{1 \leq i \leq k_{\mathcal{M}_0}}$$ and $$\{\lambda^1_i\}_{1 \leq i \leq k_{\mathcal{M}_1}}$$ are the respective eigenvalues of $$-H_0(\hat{\theta}_0)$$ and $$-H_1(\hat{\theta}_1)$$.

Proof: using the same approximation as in the derivation of the BIC for $$p(\mathcal{D}|\mathcal{M}_0)$$ and $$p(\mathcal{D}|\mathcal{M}_1)$$ yields

$$\begin{gather} \log p(\mathcal{D}|\mathcal{M}_0) = \log p(\mathcal{D}|\hat{\theta}_0,\mathcal{M}_0) + \log \pi(\hat{\theta}_0|\mathcal{M}_0)+ \frac{k_{\mathcal{M}_0}}{2} \log (2 \pi) - \frac{1}{2} \log (|-H_0(\hat{\theta}_0)|)\\ \log p(\mathcal{D}|\mathcal{M}_1) = \log p(\mathcal{D}|\hat{\theta}_1,\mathcal{M}_1) + \log \pi(\hat{\theta}_1|\mathcal{M}_1)+ \frac{k_{\mathcal{M}_1}}{2} \log (2 \pi) - \frac{1}{2} \log (|-H_1(\hat{\theta}_1)|) \end{gather}$$

Both quantities then need to be averaged over $$\langle \cdot \rangle_{p(\mathcal{D}| \theta_0^*,\mathcal{M}_0)}$$. Assuming

$$\begin{equation} \langle \log p(\mathcal{D}|\hat{\theta}_0, \mathcal{M}_0) \rangle _{p(\mathcal{D}| \theta_0^*,\mathcal{M}_0)} \approx \langle \log p(\mathcal{D}|{\theta}_0^*, \mathcal{M}_0) \rangle _{p(\mathcal{D}| \theta_0^*,\mathcal{M}_0)} \end{equation}$$

(i.e. that the maximum likelihood estimator $$\hat{\theta}_0$$ will be close to the true value $$\theta_0^*$$ from which data were generated) yields $$\langle \log p(\mathcal{D}|\hat{\theta}_0, \mathcal{M}_0) \rangle _{p(\mathcal{D}| \theta_0^*,\mathcal{M}_0)} \geq \langle \log p(\mathcal{D}|\hat{\theta}_1, \mathcal{M}_1) \rangle _{p(\mathcal{D}| \theta_0^*,\mathcal{M}_0)}$$ (under Gibbs's inequality). Furthermore, $$k_{\mathcal{M}_0} \leq k_{\mathcal{M}_1}$$ yields $$\pi(\hat{\theta}_0|\mathcal{M}_0) \geq \pi(\hat{\theta}_0|\mathcal{M}_1)$$ (these quantities do not depend on $$\mathcal{D}$$). The inequality is thus met for the first two terms on the right-hand side.

For the last two terms, if data are IID and if the number of data points $$T$$ in $$\mathcal{D}$$ is sufficiently large, the same approximation as in the derivation of the BIC can be made:

$$\frac{k_{\mathcal{M}}}{2} \log (2 \pi) - \frac{1}{2} \log (|-H(\hat{\theta})|) \approx -\frac{k_{\mathcal{M}}}{2} \log (T)$$

Since $$k_{\mathcal{M}_0} \leq k_{\mathcal{M}_1}$$, the inequality thus holds if data generated from $$\mathcal{M}_0$$ and $$\mathcal{M}_1$$ are IID.

If data are correlated, the above approximation does not hold. However, the determinant of the Hessian (which is a symmetric matrix) can be written as the product of the eigenvalues, which finally leads to the necessary condition. This inequality can also be seen as a more general version of a result presented in the following paper using less stringent approximations :

Heavens, Alan F., T. D. Kitching, and L. Verde. "On model selection forecasting, dark energy and modified gravity." Monthly Notices of the Royal Astronomical Society 380.3 (2007): 1029-1035.