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Lossless vs. lossy compression

  • Lossless compression: All information is preserved.

    • What we mostly do in IR.

  • Lossy compression: Discard some information

  • Several of the preprocessing steps can be viewed as lossy compression: case folding, stop words, stemming, number elimination.

  • Chap/Lecture 7: Prune postings entries that are unlikely to turn up in the top k list for any query.

    • Almost no loss quality for top k list.


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