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# Maximizing likelihood is equivalent to minimizing KL-Divergence

When reading Kevin Murphy’s book, I came across this statement: “… maxmizing likelihood is equivalent to minimizing $D_{KL}[P(. \vert \theta^{\ast}) \, \Vert \, P(. \vert \theta)]$, where $P(. \vert \theta^{\ast})$ is the true distribution and $P(. \vert \theta)$ is our estimate …“. So here is an attempt to prove that.

\begin{align} D_{KL}[P(x \vert \theta^*) \, \Vert \, P(x \vert \theta)] &= \mathbb{E}_{x \sim P(x \vert \theta^*)}\left[\log \frac{P(x \vert \theta^*)}{P(x \vert \theta)} \right] \\[10pt] &= \mathbb{E}_{x \sim P(x \vert \theta^*)}\left[\log \, P(x \vert \theta^*) - \log \, P(x \vert \theta) \right] \\[10pt] &= \mathbb{E}_{x \sim P(x \vert \theta^*)}\left[\log \, P(x \vert \theta^*) \right] - \mathbb{E}_{x \sim P(x \vert \theta^*)}\left[\log \, P(x \vert \theta) \right] \\[10pt] \end{align}

If it looks familiar, the left term is the entropy of $P(x \vert \theta^*)$. However it does not depend on the estimated parameter $\theta$, so we will ignore that.

Suppose we sample $N$ of these $x \sim P(x \vert \theta^*)$. Then, the Law of Large Number says that as $N$ goes to infinity:

$-\frac{1}{N} \sum_i^N \log \, P(x_i \vert \theta) = -\mathbb{E}_{x \sim P(x \vert \theta^*)}\left[\log \, P(x \vert \theta) \right]$

which is the right term of the above KL-Divergence. Notice that:

\begin{align} -\frac{1}{N} \sum_i^N \log \, P(x_i \vert \theta) &= \frac{1}{N} \, \text{NLL} \\[10pt] &= c \, \text{NLL} \\[10pt] \end{align}

where NLL is the negative log-likelihood and $c$ is a constant.

Then, if we minimize $D_{KL}[P(x \vert \theta^*) \, \Vert \, P(x \vert \theta)]$, it is equivalent to minimizing the NLL. In other words, it is equivalent to maximizing the log-likelihood.

Why does this matter, though? Because this gives MLE a nice interpretation: maximizing the likelihood of data under our estimate is equal to minimizing the difference between our estimate and the real data distribution. We can see MLE as a proxy for fitting our estimate to the real distribution, which cannot be done directly as the real distribution is unknown to us.