Cmput 650 Final Project Probabilistic Spelling Correction for Search Queries

Overview ● Motivation ● Problem Statement ● Noisy Channel Model ● EM Background ● EM for Spelling Correction

Search Query Spelling Correction ● Motivation – Over 700 M search queries made every day – 10% misspelled ● Problems – Queries are often not found in a dictionary – Many possible candidate corrections for any given misspelled query

Possible Approaches ● Naïve Method – search a dictionary for the closest match, using levenshtein edit distance – return closest match ● Better method – search a dictionary for closest matches – use levenshtein edit distance and word unigram probability to select best match

Noisy Channel Model ● Basic Noisy Channel Model – Given v, find best w ● argmax n P(w n ) = argmax n P(v|w n ) * P(w n ) ● error model: P(v|w); language model P(w) ● Why not just use Levenshtein Distance? – eg. britny -> briny vs britney ● Further Improvement – Use probabalistic edit distance (error model) and N-gram probability (language model)

Error Model P(v|w) ● Standard (Levenshtein) Edit Distance – algorithm, ins,del,sub costs, example n = length (target) m = length(source) for i = 0 to n for j = 0 to m d[i,j] = MIN(d[i-1,j] + ins-cost(targeti), d[i-1,j-1] + sub-cost(sourcej, targeti), d[i,j-1] + del-cost(sourcej) )

Better Error Model P(v|w) ● Probabilistic Edit Distance – ED proportional to probability of the edit ● Different probability/cost for each edit pair ● eg. P(e->i) > P(e->z) – How do we relate edit distance (lower is “better”) and probability (higher is “better”) ? ● d(v,w) = -log(P(v|w))

What we want ● Error Model (Unknown) – P(v|w) ● P(w): Language Model (known) – P(w) = c(w) / Σ w c(w) ● Use query logs and the language model to determine the error model

Probabilistic Edit Distance ● Determining the probabilistic edit model – Expectation Maximization ● For each query v – Determine the most likely “corrections” using the existing edit distance model and language model ● for each word within ED(x) ● candidates = args max n P(v|w n )P(w n ) ● one candidate may be the word itself – Update the edit distance model – What is EM?

Clustering and EM ● Hard Clustering (K-means)

Hard and Soft Clustering ● Soft Clustering (EM)

Expectation Maximization ● E-Step – Assign each data point to each cluster in proportion to how well it fits the cluster ● M-Step – Update the cluster centers to reflect the addition of the point

EM for Spelling Correction ● For a given query v – Find all candidate words w within ED(x); – E-Step ● For each candidate word – E[z vw ] =P(w|v)= P(v|w)P(w)/ Σ w P(v|w)P(w) – P(v|w) = Π P(ec ij ) – P(ec ij ) is the Probability of edit [letter i-> letter j]

EM for Spelling Correction ● M-Step – Given P(v) = P(e 1...e n |w)P(w) ● each e i is a single ins, del, or sub of two letters – want to adjust P(e 1 ).. P(e 2 ) accordingly – f(e i ) += P(w) – P(e i ) += f(e i ) / N ● N total number of edit operations for that letter – D(e i ) = -log(P(e i ))

M-Step ● E and M-Step working together E-Step Edit Sequences, P(ES|D) D = -log(P(l 1,l 2 ))

Results ● Example – Robert is a frequent search term, Qbert is not. – Atari makes a comeback...

Revenge of Qbert