A few days ago I found out that there had appeared lda2vec (by Chris Moody) – a hybrid algorithm combining best ideas from well-known LDA (Latent Dirichlet Allocation) topic modeling algorithm and from a bit less well-known tool for language modeling named word2vec.
And now I’m going to tell you a tale about lda2vec and my attempts to try it and compare with simple LDA implementation (I used gensim package for this). So, once upon a time…
Lda2vec absorbed the idea of “globality” from LDA. It means that LDA is able to create document (and topic) representations that are not so flexible but mostly interpretable to humans. Also, LDA treats a set of documents as a set of documents, whereas word2vec works with a set of documents as with a very long text string.
So, lda2vec took the idea of “locality” from word2vec, because it is local in the way that it is able to create vector representations of words (aka word embeddings) on small text intervals (aka windows).
Word2vec predicts words locally. It means that given one word it can predict the following word. At the same time LDA predicts globally: LDA predicts a word regarding global context (i.e. all set of documents).
Typical word2vec vector looks like dense vector filled with real numbers, while LDA vector is sparse vector of probabilities. When I speak about sparsity, I mean that most values in that vector are equal to zero.
Due to the sparsity (but not only) LDA model can be relatively easy interpreted by a human being, but it is inflexible. On the contrary (surprise!), dense word2vec vector is not human-interpretable, but very flexible (has more degrees of freedom).
The question is: how to combine these two incombinable approaches?
The author of lda2vec applies an approach almost similar to the approach from paragraph2vec (aka doc2vec), when every word-vector sums to that word’s document label.
In lda2vec, however, word2vec vectors sum to sparse “LDA-vectors”. Then, algorithm appends categorical features to these summed word+LDA vectors and estimates a multinomial mixture over the latent word topics. The resulting vector is applied to a conditional probability model to predict the final topic assignments for some set of pre-defined groupings of input documents.
Thus, when we speak about predicting words in text, we can predict the following word not only given the context of that word, like:
P(Vout|Vin)
but also considering probability of two words co-occur in certain topic and with certain categorical features, like:
P(Vout|Vin+Vdoc+Vfeature)
where
Vdoc – is sparse LDA vector
Vfeature – some categorical feature (say, zip-code)
Applying this approach to practice, it might be possible to make supervised learning models if we get very good resulting topics. We can try to use lda2vec for, say, book analysis. For every word, lda2vec sums this word’s word2vec vector to LDA-vector and then adds some known categorical features (like year or book publisher’s name). Then, lda2vec uses the resulting vector to assign the resulting LDA topics to the respective authors of the books. What’s interesting, the resulting vectors may be used to predict some author-specific parameters, like each author’s popularity.
For those of you, who doesn’t like long stories, I can concentrate all the above information into several bullets.
So, what is it all about?
And why someone should need it?
The author uses “Twenty newsgroups” sample dataset from scikit-learn python ML library (i.e. sklearn.datasets) for demonstrating the results. This dataset consists of 18000 texts from 20 different topics. All the data is split into “train” and “test” datasets. For lda2vec example the author uses the training part of the dataset.
The actual topics are as follows:
Their names are not too human-readable, but it is possible to understand what these topics are about.
Now, let’s compare the topics lda2vec produces with topics from the pure LDA algorithm (I used gensim package for this).
I evaluated the results of lda2vec and LDA using two subjective criteria (sorry, I was too lazy to do something more objective):
The author shows the following output of his lda2vec implementation in his GitHub r...:
Although the topics look dirty enough, it is possible to label some of them with real topic names. Personally I was able to assign 8 real topics to the lda2vec topics and 11 of them look ok (including those I was able to label).
And now let’s compare this results to the results of pure gensim LDA algorihm. I sketched out a simple script based on gensim LDA implementation, which conducts almost the same preprocessing and almost the same number of iterations as the lda2vec example does. You may look up the code on my GitHub account and freely use it for your purposes.
So, the LDA topics are the followng:
You may see that LDA shows almost similar results: I was able to label 8 topics and 11 of them look normal for me.
Thus, I assume current lda2vec implementation to produce good output, but it is not significantly better than the output of pure LDA (however, the results of both LDA and lda2vec may be even better if we increase the number of iterations).
Disclaimer: someone may say that the number of assigned topics as well as the number of looking-ok topics is lower (or higher), but nevertheless it is obvious that the quality of topic modeling for both tools is almost the same.
Ok, how do I shot web?
You may easily download the lda2vec implementation from its author’s GitHub repository, then extract the archive into some directory, then use Linux command shell to install the lda2vec via setup.ru:
sudo python /path-to-lda2vec-package/lda2vec/setup.py install
where
/path-to-lda2vec-package/ - is obviously the path to the unzipped lda2vec.
If you install the archive into non-standard directory (I mean that directory with all the python libraries), you will need to add the path to the lda2vec directory in sys.path. It’ll be like:
import sys
sys.path.append('/home/torselllo/lda2vec/')print sys.path>>['/usr/lib/python2.7', '/usr/local/lib/python2.7/dist-packages','/usr/lib/python2.7/dist-packages', '/usr/lib/pymodules/python2.7','/home/torselllo/lda2vec/']
And it’s done. You may now try the tool. But do not forget about the dependencies!
You need to install the following dependencies:
The most obvious way to install all the above mentioned dependencies is pip.
It is worth noting that spaCy may not be an easy thing to install. For example, after installing it with pip, you may try to start lda2vec immediately after the installation. But you’ll get the following input-output error:
>> IOError: /usr/local/lib/python2.7/dist-packages/spacy/en/data/vocab/strings.json.
If you try to find the cause of such error, you will see that there is really nothing exists in that path. Then you will probably consult official spaCy website and see that you need to install the model for English language using the following shell command:
sudo python -m spacy.en.download
Be advised that the size of the model is 519.05MB.
It looks easy but not that obvious if you just install spaCy as the dependency without visiting the official website.
Your problems may continue if the model for English language will download into wrong directory (yes, it may happen). For example, it may download into:
/usr/local/lib/python2.7/dist-packages/spacy/data/en_default-1.0.7,
while lda2vec will continue looking for the model in
/usr/local/lib/python2.7/dist-packages/spacy/en/data/
To end this story up, you may simply use the dirty hack. I mean that you can just move the model into the required directory with the mv shell command. I did it like this:
sudo mv /usr/local/lib/python2.7/dist-packages/spacy/data/en_default-1.0.7 /usr/local/lib/python2.7/dist-packages/spacy/en/data/
Now it works! And you may try the word2vec example. But remember that you need to be patient if you do not do GPU computations (which are said to be 10x times faster). In a version of lda2vec I used (as of January, 30 2016), it took me more an hour to process just 500 input documents on a machine with core i5 2 MHz processor and 2 Gb RAM.
This was a tale about the interesting approach to topic modeling named lda2vec and my attempts to try it and compare it to the simple LDA topic modeling algorithm.
Personally I find lda2vec intriguing, though not very impressive at the moment (The moment is January, 30 2016, by the way).
Comment
An important addition to the above post:
I did really cut it too far describing differencies of word2vec and LDA – in fact they are not so different nor incombinable from algorithmic point of view.
Yoav Goldberg did the perfect work explaining the way how word2vec works. And the word2vec approach can be reinterpreted as something very similar to SVD over distributional representation (see the link for details).
At the same time, LDA also relates to SVD in an alike way:
Thus, we really can think of the difference between LDA and word2vec as of the hyperparameter choice. Thererfore, the difference between LDA and word2vec I described in the above post is more like the difference between applications of these approaches - one for modeling word-document relationships and another - for modeling word-word relationships, but the basic algorithmic principles of LDA and word2vec are alike and make it possible to combine them into one hybrid algorithm.
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