Houjun Liu


perplexity is a measure of a language model’s ability to predict words.


A good language model should prefer “real” or otherwise “frequently observed” sentences. That is, it should assign lower probability to word salad.

So a good language model should assign a higher probability to the next word that actually occurs given a sequence of words.

Generally, we want the LM to assign high probability to the entire test set. However, big issue is that probability gets smaller by length of the text.

To address this, we normalize by number of words.


\begin{equation} PP (W) = P(w_1 w_2 \dots w_{n})^{-\frac{1}{N}} \end{equation}

We can also express it in terms of log likelyhood

\begin{align} PP (W) &= \prod_{t=1}^{T} \qty(\frac{1}{P_{LM}(x^{(t+1)} | x^{(t), \dots, t^{(1)}})})^{\frac{1}{T}}\\ &= \prod_{t=1}^{T} \qty( \frac{1}{\hat{\bold{y}}_{t+1}^{(t)}})^{\frac{1}{T}}\\ &= \exp \qty( \frac{1}{T} \sum_{t=1}^{T} -\log \hat{\bold{y}}_{t+1}^{(t)}) \\ &= \exp \qty(J(\theta))) \end{align}

Notably, perplexity is inverse of probability. We want the lowest entropy possible, i.e. the highest likelihood possible. Therefore, the range of perplexity is \([1, \infty]\). We therefore want to minimize perplexity.

Branching Factor

perplexity could be also considered the “weighted average Branching Factor” of the language. That is, the average number of possible next words given each of the words.

The Branching Factor is the set of possible next words that can follow a given word.


Conditioned upon previous words or current n-gram, sample from the next possible word in the distribution.

Meaning, we sample from the distribution of n-grams whose n-1 characters are known.


Out-of-sample ngrams will never be counted, no matter how truly likely.

Also, it causes perplexity problems: because you can’t divide by 0, perplexity assumes that any sequence of words should have non zero likeliness.