Today’s mathematics glossary term is another one requested by Jacob Kanev. Kaven, I learned last time, has got a blog, “Some Unconsidered Trifles”, for those interested in having more things to read. Kanev’s request this time was a term new to me. But learning things I didn’t expect to consider is part of the fun of this dance.

## Kullback-Leibler Divergence.

The Kullback-Leibler Divergence comes to us from information theory. It’s also known as “information divergence” or “relative entropy”. Entropy is by now a familiar friend. We got to know it through, among other things, the “How interesting is a basketball tournament?” question. In this context, entropy is a measure of how surprising it would be to know which of several possible outcomes happens. A sure thing has an entropy of zero; there’s no potential surprise in it. If there are two equally likely outcomes, then the entropy is 1. If there are four equally likely outcomes, then the entropy is 2. If there are four possible outcomes, but one is very likely and the other three mediocre, the entropy might be low, say, 0.5 or so. It’s mostly but not perfectly predictable.

Suppose we have a set of possible outcomes for something. (Pick anything you like. It could be the outcomes of a basketball tournament. It could be how much a favored stock rises or falls over the day. It could be how long your ride into work takes. As long as there are different possible outcomes, we have something workable.) If we have a probability, a measure of how likely each of the different outcomes is, then we have a probability distribution. More likely things have probabilities closer to 1. Less likely things have probabilities closer to 0. No probability is less than zero or more than 1. All the probabilities added together sum up to 1. (These are the rules which make something a probability distribution, not just a bunch of numbers we had in the junk drawer.)

The Kullback-Leibler Divergence describes how similar two probability distributions are to one another. Let me call one of these probability distributions p. I’ll call the other one q. We have some number of possible outcomes, and we’ll use k as an index for them. p_{k} is how likely, in distribution p, that outcome number k is. q_{k} is how likely, in distribution q, that outcome number k is.

To calculate this divergence, we work out, for each k, the number p_{k} times the logarithm of p_{k} divided by q_{k}. Here the logarithm is base two. Calculate all this for every one of the possible outcomes, and add it together. This will be some number that’s at least zero, but it might be larger.

The closer that distribution p and distribution q are to each other, the smaller this number is. If they’re exactly the same, this number will be zero. The less that distribution p and distribution q are like each other, the bigger this number is.

And that’s all good fun, but, why bother with it? And at least one answer I can give is that it lets us measure how good a model of something is.

Suppose we think we have an explanation for how something varies. We can say how likely it is we think there’ll be each of the possible different outcomes. This gives us a probability distribution which let’s call q. We can compare that to actual data. Watch whatever it is for a while, and measure how often each of the different possible outcomes actually does happen. This gives us a probability distribution which let’s call p.

If our model is a good one, then the Kullback-Leibler Divergence between p and q will be small. If our model’s a lousy one, then this divergence will be large. If we have a couple different models, we can see which ones make for smaller divergences and which ones make for larger divergences. Probably we’ll want smaller divergences.

Here you might ask: why do we need a model? Isn’t the actual data the best model we might have? It’s a fair question. But no, real data is kind of lousy. It’s all messy. It’s complicated. We get extraneous little bits of nonsense clogging it up. And the next batch of results is going to be different from the old ones anyway, because real data always varies.

Furthermore, one of the purposes of a model is to be *simpler* than reality. A model should do away with complications so that it is easier to analyze, easier to make predictions with, and easier to teach than the reality is. But a model mustn’t be so simple that it can’t represent important aspects of the thing we want to study.

The Kullback-Leibler Divergence is a tool that we can use to quantify how much better one model or another fits our data. It also lets us quantify how much of the grit of reality we lose in our model. And this is at least some of the use of this quantity.