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Spelling correction

Spelling correction. Two different methods for spelling correction Isolated word spelling correction Check each word on its own for misspelling Will not catch typos resulting in correctly spelled words, e.g., an asteroid that fell form the sky Context-sensitive spelling correction

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Spelling correction

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  1. Spelling correction • Two different methods for spelling correction • Isolated word spelling correction • Check each word on its own for misspelling • Will not catch typos resulting in correctly spelled words, e.g., an asteroid that fell form the sky • Context-sensitive spelling correction • Look at surrounding words • Can correct form/from error above

  2. Query mis-spellings • We can either • Retrieve documents indexed by the correct spelling, OR • Return several suggested alternative queries with the correct spelling • Did you mean … ?

  3. Correcting queries • First: isolated word spelling correction • Fundamental premise 1: There is a list of “correct words” from which the correct spellings come. • Fundamental premise 2: We have a way of computing the distance between a misspelled word and a correct word. • Simple spelling correction algorithm: return the “correct” word that has the smallest distance to the misspelled word. • Example: informaton → information • We can use the term vocabulary of the inverted index as the list of correct words.

  4. Isolated word correction • Given a lexicon and a character sequence Q, return the words in the lexicon closest to Q • What’s “closest”? • We’ll study several alternatives • Edit distance • Weighted edit distance • n-gram overlap

  5. Edit distance • Given two strings S1 and S2, the minimum number of basic operations to covert one to the other • Basic operations are typically character-level • Insert • Delete • Replace • E.g., the edit distance from cat to dog is 3. • Generally found by dynamic programming.

  6. Edit distance • Also called “Levenshtein distance” • See http://www.merriampark.com/ld.htm for a nice example plus an applet to try on your own

  7. Edit distance • The edit distance between string s1 and string s2 is the minimum number of basic operations to convert s1 to s2. • Levenshtein distance: The admissible basic operations are insert, delete, and replace • Levenshtein distance dog-do: 1 • Levenshtein distance cat-cart: 1 • Levenshtein distance cat-cut: 1 • Levenshtein distance cat-act: 2

  8. Levenshtein distance: Computation

  9. Levenshtein distance: algorithm

  10. Dynamic programming (Cormen et al.) • Optimal substructure: The optimal solution to the problem contains within it optimal solutions to subproblems. • Overlapping subproblems: The optimal solutions to subproblems (“subsolutions”) overlap. These subsolutions are computed over and over again when computing the global optimal solution. • Optimal substructure: We compute minimum distance of substrings in order to compute the minimum distance of the entire string. • Overlapping subproblems: Need most distances of substrings 3 times (moving right, diagonally, down)

  11. Weighted edit distance • As above, but the weight of an operation depends on the character(s) involved • Meant to capture keyboard errors, e.g. m more likely to be mis-typed as n than as q • Therefore, replacing m by n is a smaller edit distance than by q • Require weight matrix as input • Modify dynamic programming to handle weights

  12. Using edit distances • Given query, first enumerate all dictionary terms within a preset (weighted) edit distance • Then look up enumerated dictionary terms in the term-document inverted index • Slow but no real fix • How do we cut the set of candidate dictionary terms? • Here we use n-gram overlap for this

  13. n-gram overlap • Enumerate all the n-grams in the query string as well as in the lexicon • Use the n-gram index (recall wild-card search) to retrieve all lexicon terms matching any of the query n-grams • Threshold by number of matching n-grams

  14. Example with trigrams • Suppose the text is november • Trigrams are nov, ove, vem, emb, mbe, ber. • The query is december • Trigrams are dec, ece, cem, emb, mbe, ber. • So 3 trigrams overlap (of 6 in each term) • How can we turn this into a normalized measure of overlap?

  15. One option – Jaccard coefficient • A commonly-used measure of overlap • Let X and Y be two sets; then the J.C. is • Equals 1 when X and Y have the same elements and zero when they are disjoint • X and Y don’t have to be of the same size • Always assigns a number between 0 and 1 • Now threshold to decide if you have a match • E.g., if J.C. > 0.8, declare a match

  16. Problem • Can we compute Jaccard coefficient efficiently? • Hints • What about the k-grams of a given term with length l? • Set operation

  17. Context-sensitive spelling correction • Our example was: an asteroid that fell form the sky • How can we correct form here? • Ideas? • One idea: hit-based spelling correction • Retrieve “correct” terms close to each query term • for flew form munich: flea for flew, from for form, munch for munich • Now try all possible resulting phrases as queries with one word “fixed” at a time • Try query “flea form munich” • Try query “flew from munich” • Try query “flew form munch” • The correct query “flew from munich” has the most hits. • Suppose we have 7 alternatives for flew, 19 for form and 3 for munich, how many “corrected” phrases will we enumerate?

  18. Context-sensitive spelling correction • The “hit-based” algorithm we just outlined is not very efficient. • More efficient alternative: look at “collection” of queries, not documents

  19. General issues in spelling correction • User interface • automatic vs. suggested correction • Did you mean only works for one suggestion. • What about multiple possible corrections? • Tradeoff: simple vs. powerful UI • Cost • Spelling correction is potentially expensive. • Avoid running on every query? • Maybe just on queries that match few documents. • Guess: Spelling correction of major search engines is efficient enough to be run on every query.

  20. Soundex • Soundex is the basis for finding phonetic (as opposed to orthographic) alternatives. • Example: chebyshev / tchebyscheff • Algorithm: • Turn every token to be indexed into a 4-character reduced form • Do the same with query terms • Build and search an index on the reduced forms

  21. Soundex algorithm • 1 Retain the first letter of the term. • 2 Change all occurrences of the following letters to ’0’ (zero): ’A’, E’, ’I’, ’O’, ’U’, ’H’, ’W’, ’Y’ • 3 Change letters to digits as follows: • B, F, P, V to 1 • C, G, J, K, Q, S, X, Z to 2 • D,T to 3 • L to 4 • M, N to 5 • R to 6 • 4 Repeatedly remove one out of each pair of consecutive identical digits • 5 Remove all zeros from the resulting string; pad the resulting string with trailing zeros and return the first four positions, which will consist of a letter followed by three digits

  22. Example: Soundex of HERMAN • Retain H • ERMAN → 0RM0N • 0RM0N → 06505 • 06505 → 06505 • 06505 → 655 • Return H655 • Will HERMANN generate the same code?

  23. Compute soundex code of your first name.

  24. How useful is Soundex? • Not very – for information retrieval • Ok for “high recall” tasks in other applications (e.g., Interpol) • Zobel and Dart (1996) suggest better alternatives for phonetic matching in IR.

  25. The complete search system

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