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So I am working on a problem where I want to extract a set of LDA topics from one corpus, and then compare the distribution of those topics in other corpora. So basically I want to lock-in the topics and then get a sense of how similar or different another corpus is from the original corpus. I was hoping that someone could tell me the tool or approach to do this type of comparison?

My particular application has to do with comparing local versus national newspapers. I have a corpus of national newspaper articles and I have already used gensim to extract the topics. Now I have corpora of local newspapers captured during the same period of time. So I want to compare the distribution of identical topics in the national newspaper versus the local newspapers. Of course, I would also like to look at the structure of the topic in both the national and local corpora (such as the change in probability of co-occurrence of words for the same topic in the two different corpora).

I looked around in the R topicmodels and the python gensim packages, but had no luck. Any suggestions?

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You have to train your model, get the topics distribution for both the corpus you want to compare and then you need to choose a metric to compare them. For example, the topic distributions are vectors, and you can use the euclidian distance between them as an indicator of the difference between the documents.

EDIT - (example)

With gensim, you'll have to do something like that:

#Train your LDA model    
lda = LdaModel(national_corpus, num_topics=10)

# Get the mean of all topic distributions in one corpus
national_topic_vectors = []
for newspaper in national_corpus:
    national_topic_vectors.append(lda[newspaper])
national_average = numpy.average(numpy.array(national_topic_vectors), axis=0)

# Get the mean of all topic distributions in another corpus
regional_topic_vectors = []
for newspaper in regional_corpus:
    regional_topic_vectors.append(lda[newspaper])
regional_average = numpy.average(numpy.array(regional_topic_vectors), axis=0)

# Calculate the distance between the distribution of topics in both corpora
difference_of_distributions = numpy.linalg.norm(national_average - regional_average)
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  • $\begingroup$ Say, do you know of any tutorials or code samples that would show me how to do this? I have read though the gensim tutorials, but I did not see a way to do what you say. $\endgroup$ – krishnab Jun 12 '14 at 5:27
  • $\begingroup$ I've added an example, but it will always depend on how you've formatted your corpus (BoW, sequence of tokens, etc) $\endgroup$ – Jundiaius Jun 12 '14 at 11:44
  • $\begingroup$ Oh this is excellent. Thanks. Yeah, now I see how to use the outputs from the LDA model to get the summaries I am looking for. Thanks so much. $\endgroup$ – krishnab Jun 14 '14 at 23:34
  • $\begingroup$ Is there a difference between getting the mean of all topic distributions in one corpus and just combining all documents in a corpus into one corpus and then doing lda[corpus]? $\endgroup$ – user0 Jun 27 '15 at 23:02

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