【推荐】LSI(latent semantic indexing) 完美教程
"instead of lecturing about SVD I want to show you how things work --step by step"
-- 如果大家认同这句话的话,Dr. E. Garcia写的此教程就是最适合你阅读的LSI / LSA教程。
原文比较长,直接贴链接了:
http://www.miislita.com/information-retrieval-tutorial/svd-lsi-tutorial-1-understanding.html
若觉得原文太长,还可以看Garcia写的精简版:
Latent Semantic Indexing (LSI) Fast Track Tutorial
Singular Value Decomposition (SVD) Fast Track Tutorial
摘录部分内容:
一、常见的对LSI的不正确认识:
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).
Pasted from <http://www.miislita.com/information-retrieval-tutorial/svd-lsi-tutorial-1-understanding.html>
二、LSI本质上识别了以文档为单位的second-order co-ocurrence的单词并归入同一个子空间。因此:
1)落在同一子空间的单词不一定是同义词,甚至不一定是在同情景下出现的单词,对于长篇文档尤其如是。
2)LSI根本无法处理一词多义的单词(多义词),多义词会导致LSI效果变差。
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:
A and B do not co-occur.
X mentions terms A and C
Y mentions terms B and C.
:. A---C---B
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.
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.
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.
Pasted from <http://www.miislita.com/information-retrieval-tutorial/svd-lsi-tutorial-1-understanding.html>