N-Gram Language Models

Markov Assumption

This is all based off the Markov Assumption: The probability of a word only depends on a fix number of previous words. This means N-Gram language models are not that good for long contexts.

What:

Before Transformers or LSTMs, we had N-Gram language models. They were quick, explainable, efficient statistical language models.

It predicts the probability of a word given its previous words. (Conditional Probability)

How?

First, N-Grams:

A sequence of n words from a given text.

• Unigram (n=1): Each word is independent (e.g., “The”, “dog”, “runs”).
• Bigram (n=2): Considers pairs of words (e.g., “The dog”, “dog runs”).
• Trigram (n=3): Uses three-word sequences (e.g., “The dog runs”).

Predicting The Next Word:

In essence, we’re trying to predict the $n^{\text{th}}$ word given the words before it.
$P(w_n|w_{n-1},w_{n-2},...,w_{n-k+1})$

How?

We use the formula below.
$P(w_n|w_{n-1})=\frac{C(w_{n-1},w_n)}{C(w_{n-1})}$
where:

• $C(w_{n-1},w_n)$ is the count of the word sequence $(w_{n-1},w_n)$.
• $C(w_{n-1})$ is the count of the preceding word $w_{n-1}$.

What if either are 0?

Often, with larger contexts / corpuses, you either denominator or numerator will be zero. We need to (similarly to Naive Bayes) add smoothing. How? Couple ways:

1. Backoff Smoothing

What if we use less context when we’ve never seen the full thing before?
$P(\text{mat}|\text{cat on a})=\frac{N(\text{cat on a mat})}{N(\text{on a mat})}\text{What if count of "on a mat" is zero?!?}$
We would then check the count of “on a”.
$P(\text{mat}|\text{cat on a})\approx P(\text{mat}|\text{on a})$
If still zero, we would check for “a”
$P(\text{mat}|\text{on a})\approx P(\text{mat}|\text{a})$
If still zero, we would just take the unigram probability for the word itself.
$P(\text{mat}|\text{a})\approx P(\text{mat})$

2. Linear Interpolation:

We would mix the all of the probabilities of the unigram, bigram, trigram etc. The exact weighting of each (${\lambda }_n$) would be learned (and the sum of all weights adds up to 1).

$P(\text{mat}|\text{cat on a})=\frac{N(\text{cat on a mat})}{N(\text{on a mat})}\text{What if count of "on a mat" is zero?!?}$
$\hat{P}(\text{mat}|\text{cat on a})\approx {\lambda }_{3}P(\text{mat}|\text{cat on a})+{\lambda }_{2}P(\text{mat}|\text{on a})+{\lambda }_{1}P(\text{mat}|\text{a})+{\lambda }_{0}P(\text{mat})$

3. Laplace Smoothing

Also used in Naive Bayes, we simply add $1$ (or a small $\delta$) to every single count. That’s it lol. It works surprisingly well.