LDA with Gensim in NLP - ML Experiment: Train & Evaluate

Experiment - LDA with Gensim

Problem:We want to find topics in a collection of text documents using LDA (Latent Dirichlet Allocation) with Gensim.

Current Metrics:Coherence score: 0.35

Issue:The model shows low coherence score, indicating topics are not very meaningful or clear.

Your Task

Improve the coherence score to above 0.45 by tuning the number of topics and passes without overfitting.

Use Gensim's LdaModel only.

Do not change the preprocessing steps.

Keep the number of topics between 2 and 10.

Hint 1

Hint 2

Hint 3

Solution

NLP

import gensim
from gensim import corpora
from gensim.models.ldamodel import LdaModel
from gensim.models.coherencemodel import CoherenceModel

# Sample documents
texts = [
    ['human', 'interface', 'computer'],
    ['survey', 'user', 'computer', 'system', 'response', 'time'],
    ['eps', 'user', 'interface', 'system'],
    ['system', 'human', 'system', 'eps'],
    ['user', 'response', 'time'],
    ['trees'],
    ['graph', 'trees'],
    ['graph', 'minors', 'trees'],
    ['graph', 'minors', 'survey']
]

# Create dictionary and corpus
id2word = corpora.Dictionary(texts)
corpus = [id2word.doc2bow(text) for text in texts]

# Train LDA model with tuned parameters
lda_model = LdaModel(corpus=corpus,
                     id2word=id2word,
                     num_topics=4,
                     random_state=100,
                     update_every=1,
                     chunksize=10,
                     passes=20,
                     alpha='auto',
                     per_word_topics=True)

# Compute coherence score
coherence_model_lda = CoherenceModel(model=lda_model, texts=texts, dictionary=id2word, coherence='c_v')
coherence_lda = coherence_model_lda.get_coherence()

print(f"Coherence Score: {coherence_lda:.4f}")

Increased number of passes from default to 20 to allow more training iterations.

Set number of topics to 4 to better capture distinct themes.

Used 'auto' alpha to let model optimize topic distribution sparsity.

Results Interpretation

Before tuning: Coherence score = 0.35
After tuning: Coherence score = 0.52

Increasing passes and adjusting the number of topics helps the LDA model find clearer, more meaningful topics, improving coherence.

Bonus Experiment

Try using a different coherence measure like 'u_mass' and compare results.

💡 Hint

Change the coherence parameter in CoherenceModel to 'u_mass' and observe how scores differ.

Practice

1. What is the main purpose of using LDA (Latent Dirichlet Allocation) with Gensim in NLP?

easy

A. To find hidden topics in a collection of documents

B. To translate text from one language to another

C. To count the frequency of words in a document

D. To generate new sentences based on input text

Solution

1. Step 1: Understand LDA's goal

   LDA is a topic modeling technique used to discover hidden topics in text data.

2. Step 2: Match with Gensim usage

   Gensim's LDA implementation helps find these hidden topics from document collections.

3. Final Answer:

   To find hidden topics in a collection of documents -> Option A

4. Quick Check:

   LDA purpose = find hidden topics [OK]

Hint: LDA = discover hidden themes in text collections [OK]

Common Mistakes:

• Confusing LDA with translation or text generation
• Thinking LDA counts word frequency only
• Assuming LDA summarizes text instead of finding topics

2. Which of the following is the correct way to create a Gensim dictionary from tokenized documents stored in texts?

easy

A. dictionary = gensim.make_dictionary(texts)

B. dictionary = gensim.Dictionary(texts)

C. dictionary = gensim.corpora.Dictionary(texts)

D. dictionary = gensim.create_dictionary(texts)

Solution

1. Step 1: Recall Gensim dictionary creation syntax

   The correct method is gensim.corpora.Dictionary() which takes tokenized texts.

2. Step 2: Check options for exact match

   Only dictionary = gensim.corpora.Dictionary(texts) uses the full correct syntax with gensim.corpora.Dictionary.

3. Final Answer:

   dictionary = gensim.corpora.Dictionary(texts) -> Option C

4. Quick Check:

   Correct dictionary syntax = dictionary = gensim.corpora.Dictionary(texts) [OK]

Hint: Use gensim.corpora.Dictionary for token lists [OK]

Common Mistakes:

• Omitting 'corpora' module in gensim
• Using non-existent functions like make_dictionary
• Confusing dictionary creation with corpus creation

3. Given the code snippet below, what will be the output of print(ldamodel.print_topics(num_topics=2))?

import gensim
from gensim import corpora
texts = [['apple', 'banana', 'apple'], ['banana', 'orange'], ['apple', 'orange', 'banana']]
dictionary = corpora.Dictionary(texts)
corpus = [dictionary.doc2bow(text) for text in texts]
ldamodel = gensim.models.LdaModel(corpus, num_topics=2, id2word=dictionary, passes=10, random_state=42)
print(ldamodel.print_topics(num_topics=2))

medium

A. Empty list because no topics were found

B. [('0', '0.5*"apple" + 0.3*"banana" + 0.2*"orange"'), ('1', '0.6*"banana" + 0.4*"orange"')]

C. SyntaxError due to missing import of LdaModel

D. A list of tuples showing topic IDs and top words with weights

Solution

1. Step 1: Understand print_topics output

   print_topics returns a list of tuples with topic IDs and top words with weights as strings.

2. Step 2: Analyze code correctness

   Code imports gensim and corpora correctly, creates dictionary and corpus, trains LDA model, so output is topic list, not error or empty.

3. Final Answer:

   A list of tuples showing topic IDs and top words with weights -> Option D

4. Quick Check:

   print_topics output = topic list [OK]

Hint: print_topics returns topic-word lists, not exact strings [OK]

Common Mistakes:

• Expecting exact word weights as fixed numbers
• Assuming missing import causes error (gensim.models is imported)
• Thinking no topics found means empty list

4. You run the following code but get an error: AttributeError: 'LdaModel' object has no attribute 'show_topics'. What is the likely cause?

ldamodel = gensim.models.LdaModel(corpus, num_topics=3, id2word=dictionary)
print(ldamodel.show_topics())

medium

A. The dictionary was not created properly

B. Using an outdated Gensim version where show_topics is not available

C. The corpus variable is empty or None

D. Missing the 'passes' parameter in LdaModel initialization

Solution

1. Step 1: Identify error meaning

   AttributeError means the method show_topics does not exist on the LdaModel object.

2. Step 2: Check common causes

   Older Gensim versions did not have show_topics method; newer versions do. Missing passes or empty corpus cause different errors.

3. Final Answer:

   Using an outdated Gensim version where show_topics is not available -> Option B

4. Quick Check:

   AttributeError on show_topics = outdated Gensim [OK]

Hint: Check Gensim version if method not found error occurs [OK]

Common Mistakes:

• Assuming missing passes causes AttributeError
• Thinking empty corpus causes this error
• Blaming dictionary creation for method missing

5. You want to improve your LDA model's topic quality using Gensim. Which combination of actions is best?

1. Increase the number of passes during training
2. Remove very common words (stopwords) before training
3. Use a very large number of topics (e.g., 100) regardless of data size
4. Filter out words that appear in too few or too many documents

hard

A. Apply steps 1, 2, and 4 to improve model quality

B. Only increase passes (step 1) is enough for better topics

C. Use a very large number of topics (step 3) for best results

D. Remove stopwords (step 2) and increase topics (step 3) only

Solution

1. Step 1: Understand passes effect

   More passes let the model learn better from data, improving topic quality.

2. Step 2: Understand preprocessing impact

   Removing stopwords and filtering rare/common words reduces noise and improves topics.

3. Step 3: Avoid too many topics

   Using too many topics without enough data causes poor, fragmented topics.

4. Final Answer:

   Apply steps 1, 2, and 4 to improve model quality -> Option A

5. Quick Check:

   Good LDA = passes + clean data + filter words [OK]

Hint: More passes + clean data + filter words = better topics [OK]

Common Mistakes:

• Thinking more topics always improves quality
• Ignoring data cleaning steps
• Believing passes alone fix poor topics