【推薦】LSI(latent semantic indexing) 完美教程

"instead of lecturing about SVD I want to show you how things work --step by step"?

-- 如果大家認同這句話的話，Dr. E. Garcia寫的此教程就是最適合你閱讀的LSI / LSA教程。

原文比較長，直接貼鏈接了：

http://www.miislita.com/information-retrieval-tutorial/svd-lsi-tutorial-1-understanding.html

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若覺得原文太長，還可以看Garcia寫的精簡版：

Latent Semantic Indexing (LSI) Fast Track Tutorial
Singular Value Decomposition (SVD) Fast Track Tutorial

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摘錄部分內(nèi)容：

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一、常見的對LSI的不正確認識：

1） is theming (analysis of themes).

2） is used by search engines to find all the nouns and verbs, and then associate them with related (substitution-useful) nouns and verbs.

3） allows search engines to "learn" which words are related and which noun concepts relate to one another.

4） is a form of on-topic analysis (term scope/subject analysis).can be applied to collections of any size.

5） has no problem addressing polysemy (terms with different meanings).

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Pasted from <http://www.miislita.com/information-retrieval-tutorial/svd-lsi-tutorial-1-understanding.html>

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二、LSI本質(zhì)上識別了以文檔為單位的second-order co-ocurrence的單詞并歸入同一個子空間。因此：

1）落在同一子空間的單詞不一定是同義詞，甚至不一定是在同情景下出現(xiàn)的單詞，對于長篇文檔尤其如是。

2）LSI根本無法處理一詞多義的單詞（多義詞），多義詞會導致LSI效果變差。

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A persistent myth in search marketing circles is that LSI grants contextuality; i.e., terms occurring in the same context. This is not always the case. Consider two documents X and Y and three terms A, B and C and wherein:

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A and B do not co-occur.

X mentions terms A and C

Y mentions terms B and C.

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:. A---C---B

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The common denominator is C, so we define this relation as an in-transit co-occurrence since both A and B occur while in transit with C. This is called second-order co-occurrence and is a special case of high-order co-occurrence.

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However, only because terms A and B are in-transit with C this does not grant contextuality, as the terms can be mentioned in different contexts in documents X and Y. For example, this would be the case of X and Y discussing different topics. Long documents are more prone to this.

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Even if X and Y are monotopic thesemight be discussing different subjects. Thus, it would be fallacious to assume that high-order co-occurrence between A and B while in-transit with C equates to a contextuality relationship between terms. Add polysemy to this and the scenario worsens, as LSI can fail to address polysemy.

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Pasted from <http://www.miislita.com/information-retrieval-tutorial/svd-lsi-tutorial-1-understanding.html>

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