How do I find Coherence Score for LSA and LDA for SkLearn Models?

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I want to compare coherence scores for LSA and LDA models.

LSA model

lsa_model = TruncatedSVD(n_components=20, algorithm='randomized', n_iter=40, random_state=5000)

lsa_top=lsa_model.fit_transform(vect_text)

LDA model

lda_model=LatentDirichletAllocation(n_components=20,learning_method='online',random_state=42,max_iter=1)

Can someone please help me calculate the coherence scores of these 2 models?

Thank you in advance!

topic-modeling text-mining nlp text python
1 Answers

I'm using sklearn TfidfVectorizer combined with TruncatedSVD to find best topics for my corpus. Could not find built in coherence for TruncatedSVD and had to implement my own. The code is based on this article:

http://qpleple.com/topic-coherence-to-evaluate-topic-models/

I've decided to stick to intrinsic UMass measure since it it relatively easy to implement. Support methods are:

def get_umass_score(dt_matrix, i, j):
    zo_matrix = (dt_matrix > 0).astype(int)
    col_i, col_j = zo_matrix[:, i], zo_matrix[:, j]
    col_ij = col_i + col_j
    col_ij = (col_ij == 2).astype(int)    
    Di, Dij = col_i.sum(), col_ij.sum()    
    return math.log((Dij + 1) / Di)

def get_topic_coherence(dt_matrix, topic, n_top_words):
    indexed_topic = zip(topic, range(0, len(topic)))
    topic_top = sorted(indexed_topic, key=lambda x: 1 - x[0])[0:n_top_words]
    coherence = 0
    for j_index in range(0, len(topic_top)):
        for i_index in range(0, j_index - 1):
            i = topic_top[i_index][1]
            j = topic_top[j_index][1]
            coherence += get_umass_score(dt_matrix, i, j)
    return coherence

def get_average_topic_coherence(dt_matrix, topics, n_top_words):
    total_coherence = 0
    for i in range(0, len(topics)):
        total_coherence += get_topic_coherence(dt_matrix, topics[i], n_top_words)
    return total_coherence / len(topics)

The usage:

for n_topics in range(5, 1000, 50):
    svd = TruncatedSVD(n_components=n_topics, n_iter=7, random_state=42)
    svd.fit(tfidf_matrix)
    avg_coherence = get_average_topic_coherence(tfidf_matrix, svd.components_, 10)
    print(str(n_topics) + " " + str(avg_coherence))

Output:

5 -72.44478726897546
55 -86.18040144608892
105 -88.9175058514422
155 -90.3841147807378
205 -91.83948259181923
255 -92.01751480271953 < best
305 -90.73603639282118
355 -89.85740639388695
405 -89.41916273620417
455 -87.66472648614531
505 -85.06725618307024
555 -81.1419066684933
605 -77.03963739283286
655 -73.04509144854593
705 -69.84849596265884
755 -68.01357538571055
805 -67.48039395600706
855 -67.53091204608572
905 -67.23467504644942
955 -66.86079451952988

The lower UMass coherence - the better. In my case 255 topics is the best fit for my corpus. And I used 10 most relevant words for a topic - you can use your number. You will get different numbers, but optimal number of topics (SVD components) will generally be the same.

I'm using TF-IDF vectors, but this coherence should work on any term frequency based approach (ex. BOW)

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