# Review some Information Theory

Here are my note on some concepts from self-study on Information Theory:

The Shannon information content:

$h(x) = \log_2 \frac{1}{p(x)}$

This is a measurement of the information content of the event x = a. For example, when we flip a coin, an event of landing a head is 0.5, then h(x=’head’) = 1. It means that we only need one bit for this event. E.g. If we landed a tail, we know that it won’t be a head. One bit is sufficient.

Entropy: an average of Shannon information:

$H(X) = -\sum_x p(x) \log_2 p(x) = E[- \log_2 p(x) ]$

We consider all events and average out all the Shannon information. Entropy can be thought as an uncertainty. The higher entropy, the more uncertainty of the event. This implies that the Entropy will be maximum when p(x) is a uniform distribution since we can’t make any prediction.

The Relative entropy (KL Divergence)

$KL(P||Q) = \sum_x p(x) \log_2 \frac{p(x)}{q(x)} = E[\log_2 \frac{p(x)}{q(x)}]$

KL(P||Q) is always non-negative (Check Gibbs’ inequality). This term will show up often in many machine learning models. It measures how much distribution q(x) diverges from p(x).

The Conditional entropy of X given Y:

This is an average uncertainty that remains about x when y is known. If I know y, then how much I still don’t know about x.

$H(X|Y=b_k) = - \sum p(x|y=b_k) \log_2 p(x|y=b_k)$

If we average over y:

$latext H(X|Y) = \sum_y p(y) [-\sum_x p(x|y) \log_2 p(x|y) ] = -sum_{x,y} p(x,y) \log p(x|y)$

Chain rule:

$h(x,y) = h(x) + h(y|x)$

Information content of x and y is the information content of x plus information coent of y given x.

Entropy Chain rule:

$H(X,Y) = H(x) + H(Y|X)$

An uncertainty about X and Y is the uncertainty of X plus uncertainty of Y given X.

Mutual Information:

This term appears often in information retrieval and some machine learning models. This term measures the average reduction in uncertainty about x that results from learning the value of y. E.g. How much I will learn about x once I know about y ? What is an average amount of information that y conveys about x?

$I(X; Y) = H(X) - H(X|Y)$

$I(X; Y) = I(Y; X) \ge 0$

When we add more data, we always decrease uncertainty.

$H(X,Y) \le H(X) + H(Y)$

$H(Y|X) = H(X,Y) - H(X) \le H(Y) + H(X) - H(X) \le H(Y)$

References:

[1] MacKay, David JC. Information theory, inference and learning algorithms. Cambridge university press, 2003.

# Entropy in Variational Inference

In Variational Inference, the evidence lowerbound (ELBO) is defined as:

$\log p(x) = \log \int_z p(x, z) dz = \log E_{q(z)}[\frac{p(x,z)}{q(z)}]$
$>= E_{q(z)}[\log p(x, z) ]- E_{q(z)}[ \log q(z) ]$

At this point, I usually pay attention to the first term, the expected log-likelihood and treat the second term, entropy, as extra baggage. It makes sense that the entropy term is less relevant especially in mean-field approximation because when the complicated approximate distribution is now just a factored of much simpler approximate distributions, $q(z) = \prod_i q(z_i)$, the entropy term is just a sum of entropy for each hidden variables.

The entropy term shows up again when I worked on the Variational Autoencoder formulation. The entropy term is merge with a prior and result in a relative entropy:

$\log p(x) >= E_{q(z)}[\log p(x| z) ] + E_{q(z)}[\log p(z)]- E_{q(z)}[ \log q(z) ]$
$= E_{q(z)}[\log p(x| z) ] - D_{kl}(q(z)||p(z))$

When I pay closer attention, the relative entropy represents a gap (slack) between the lowerbound and the log-likelihood. The high relative entropy means that I badly approximate the true posterior.

When I studied information theory more thoroughly, the entropy can be viewed as the uncertainty of the event of interest. The more uncertainty, the higher entropy because we need to add extra bits to encode these unknown. Then, the entropy in the ELBO could imply how useful the variational distribution. If $q(z)$ is a uniform distribution, then it is useless because we cannot make any good prediction. Thus, we want $q(z)$ to be less flat which automatically means more predictable and lower entropy.

I think I need to be able to look from information theory perspective to get a better sense of how things work in Variational Inference. This knowledge will definitely be helpful for my future research.