Inside Outside algorithm

This is a Method page for the Inside-outside algorithm.

Background

The inside-outside algorithm is a way of estimating probabilities in a PCFG. It is first introduced [| Baker, 1979]. The inside outside algorithm is in fact a generalization of the forward-backward algorithm (for hidden Markov models) to PCFGs.

Algorithm

The algorithm is a dynamic programming algorithm that is often used with chart parsers to estimate expected production counts. Here, we assume the grammar $G$ is of Chomsky Normal Form.

The algorithm works by computing 2 probabilities for each nonterminal $A$ and span $i,j$.

Inside probabilities

The inside probability is defined as $\alpha (A,i,j)=P(A{\overset {*}{\Rightarrow }}w_{i}...w_{j}|G,\mathbf {w} )$, which is the probability of a nonterminal $A$ generating the word sequence $w_{i}$ to $w_{j}$.

The inside probability can be calculated recursively with the following recurrence relation:

$\alpha (A,i,j)=\sum _{B,C}\sum _{i\leq k\leq j}p(A\rightarrow BC)\alpha (B,i,k)\alpha (C,k+1,j)$

Intuitively, this can be seen as computing the sum over all possible ways of building trees rooted by $A$ and generating the word span $i,j$.

For the base case, it is simply $\alpha (A,i,i)=p(A\rightarrow w_{i})$.

Outside counts

The outside probability is defined as $\beta (A,i,j)=P(S{\overset {*}{\Rightarrow }}w_{1},...,w_{i-1},A,w_{j+1},...,w_{n})$, which is the probability of generating a parse tree spanning the entire sentence that uses nonterminal $A$ to span $i,j$.

The reccurrence relation is thus:

$\beta (A,i,j)=\sum _{B,C}\sum _{1\leq k<i}p(B\rightarrow CA)\alpha (C,k,i-1)\beta (B,k,j)+\sum _{B,C}\sum _{j<k\leq n}p(B\rightarrow AC)\alpha (C,j+1,k)\beta (B,i,k)$

The first term is basically considering all ways of generating trees where $A$ is used as a right subtree, and vis a vis for the second term.

Dynamic programming: Putting them together

In a standard EM framework, we would want to compute for each production rule, the expected number of times it is used for a given sentence, which we can compute by summing over the counts of using the production for all possible spans (and separation points)

$count(A\rightarrow BC)={\dfrac {p(A\rightarrow BC)}{\alpha (S,1,n)}}\sum _{1\leq i\leq j\leq k\leq n}\beta (A,i,k)\alpha (B,i,j)\alpha (C,j+1,k)$

Complexity

For a given sentence of length $n$ and grammar $G$, the inside outside algorithm is $O(Gn^{3})$