Latent Semantic Analysis Tutorial | Hacker News Reader

Latent Semantic Analysis Tutorial

puffinwarellc.com

63 pointsrouli8 comments

State of the art implementation, using random projection for reasonably accurate yet hundreds to thousands of times faster: http://radimrehurek.com/gensim/

Random Projection is something you should be aware of if you do any kind of large dimensional modeling. It is magic.

Also, random indexing, which is in its theoretic essence the same thing: http://www.sics.se/~mange/random_indexing.html

Truncated SVD has been a wonderful tool for "cleaning up" pairwise cosine similarity data for text document comparisons, graph/network building (for a visual representation of entities represented by documents, embedded in something like Gephi/Sigma.js/D3), and for item-based recommendation systems.

The biggest problems I then run into involves choosing a "k" (the dimensions allowed in your truncation). Have had some thoughts about training this unsupervised method (providing labeled data for what "oughta" be the top nearest neighbors for this particular entity, and optimizing toward that) or building an ensemble method on top of many SVD'd truncated vector spaces (though the combination method is unclear to me-- pick kNN from a linear combination of each model's outcomes? Pick the intersection of each method's k nearest neighbors?)

To novices looking at this tutorial: NumPy's a wonderful tool for small toy examples, but at a certain scale you will depend heavily on the sparse matrix formats provided for you by SciPy. (That and random projections should curb any memory problems you'll run into for many vector space-based problems, short of operating at a Google/Yahoo scale, or if your target's TBs of logging data).

Visualising multi-dimensional data is indeed, tricky. The best tool I have found so far for word representations similar to LSA has been t-SNE:

http://homepage.tudelft.nl/19j49/t-SNE.html

In some cases what I get isn't all that much better than simply using PCA, but overall t-SNE is superior. Although t-SNE is dreadfully slow... Below is a link to an implementation used for text and I can highly recommend the original paper on t-SNE:

https://github.com/turian/textSNE

FYI

What are the differences among latent semantic analysis (LSA), latent semantic indexing (LSI), and singular value decomposition (SVD)?

http://stats.stackexchange.com/questions/4735/what-are-the-d...

Also: "LSA was patented in 1988 (US Patent 4,839,853) by Scott Deerwester, Susan Dumais, George Furnas, Richard Harshman, Thomas Landauer, Karen Lochbaum and Lynn Streeter."

https://en.wikipedia.org/wiki/Latent_semantic_analysis

So the patent is expired now, right?

Also here is a nice paper to get started with LSI : http://www2.denizyuret.com/ref/berry/berry95using.pdf

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