nmf-topic-modeling

name	nmf-topic-modeling
description	Use when discovering latent themes in a text corpus, performing unsupervised topic extraction, comparing NMF vs LDA results, or needing to find hidden connections across documents. Triggers include topic modeling, theme discovery, TF-IDF vectorization, document clustering, coherence scoring, and cross-topic analysis.

NMF Topic Modeling

Overview

Non-negative Matrix Factorization (NMF) decomposes a TF-IDF document-term matrix into two non-negative matrices: W (document-topic) and H (topic-term). The non-negativity constraint produces additive, interpretable topics where each document is a weighted mixture of themes and each theme is a weighted mixture of terms. NMF typically produces more coherent, less overlapping topics than LDA, especially on shorter texts.

When to Use

• Discovering latent themes across a corpus of documents (posts, comments, articles, reviews)
• Finding hidden connections between seemingly unrelated content areas
• Corpus has 100+ documents with substantive text (not just titles or single sentences)
• Need interpretable, distinct topics with minimal overlap
• Working with TF-IDF weighted text (NMF strength vs LDA's bag-of-words)

When NOT to use:

• Corpus has fewer than 50 documents -- too sparse for meaningful factorization
• Documents are extremely short (under 10 words each) without enrichment strategies
• You need probabilistic topic distributions with uncertainty estimates (use LDA instead)
• A single definitive classification is needed (use supervised methods instead)

digraph nmf_vs_lda {
    rankdir=LR;
    "Short texts or\nneed distinct topics?" [shape=diamond];
    "Need probabilistic\ndistributions?" [shape=diamond];
    "Use NMF" [shape=box];
    "Use LDA" [shape=box];
    "Consider BERTopic\nor Top2Vec" [shape=box];

    "Short texts or\nneed distinct topics?" -> "Use NMF" [label="yes"];
    "Short texts or\nneed distinct topics?" -> "Need probabilistic\ndistributions?" [label="no"];
    "Need probabilistic\ndistributions?" -> "Use LDA" [label="yes"];
    "Need probabilistic\ndistributions?" -> "Consider BERTopic\nor Top2Vec" [label="no, need\nsemantic depth"];
}

Quick Reference

Parameter	Recommended Starting Value	Notes
max_df	0.95	Remove terms in >95% of docs (corpus-specific stopwords)
min_df	2-5	Remove terms appearing in fewer than N docs
max_features	1000-5000	Cap vocabulary size; start at 2000
n_components (k)	8-15	Select via coherence scoring; start with 10
init	"nndsvda"	Deterministic initialization; most stable
solver	"mu" (Multiplicative Update)	Use with beta_loss="kullback-leibler" for sparse topics
alpha_W / alpha_H	0.1	L1/L2 regularization; increase for sparser topics
l1_ratio	0.5	Balance between L1 (sparsity) and L2 (smoothness)
random_state	42	Reproducibility
Top N terms per topic	10-15	For display and coherence evaluation

Workflow

Copy this checklist and track progress:

NMF Topic Modeling Progress:
- [ ] Step 1: Validate corpus size and quality
- [ ] Step 2: Preprocess text and build TF-IDF matrix
- [ ] Step 3: Select number of topics (k) via coherence sweep
- [ ] Step 4: Fit NMF model with chosen k
- [ ] Step 5: Extract and label topics (human review)
- [ ] Step 6: Compute document-topic distributions
- [ ] Step 7: Identify cross-topic connections
- [ ] Step 8: Compare with LDA (optional validation)
- [ ] Step 9: Write findings to docs/analysis/05-nmf-topic-modeling.md

Step 1: Validate Corpus Size and Quality

Before running NMF, verify the corpus meets minimum requirements.

Minimum corpus thresholds:

Corpus Size	Recommendation
< 50 documents	Insufficient. Combine with related corpora or use keyword extraction instead.
50-100 documents	Marginal. Reduce k to 3-5 topics. Expect low coherence. Flag results as exploratory.
100-500 documents	Viable. Target 5-10 topics.
500-2000 documents	Good. Target 8-15 topics.
2000+ documents	Excellent. Can support 15-25+ topics if coherence holds.

Quality checks:

• Average document length should be 20+ words after preprocessing
• Remove empty documents, duplicates, and boilerplate-only entries
• If >50% of documents are under 10 words, consider concatenation strategies (e.g., group by author, thread, or time window)

If corpus is insufficient: Report this finding in the output document rather than forcing analysis on inadequate data. State the corpus size, why it is insufficient, and what minimum size would be needed.

Step 2: Preprocess Text and Build TF-IDF Matrix

from sklearn.feature_extraction.text import TfidfVectorizer
import re

def preprocess_text(text):
    """Clean text for NMF. Extend stopwords iteratively."""
    text = text.lower()
    text = re.sub(r'http\S+|www\.\S+', '', text)  # Remove URLs
    text = re.sub(r'[^a-zA-Z\s]', ' ', text)       # Keep only letters
    text = re.sub(r'\s+', ' ', text).strip()         # Normalize whitespace
    return text

documents = [preprocess_text(doc) for doc in raw_documents]

# Start with sklearn's English stopwords, then iterate
custom_stopwords = list(TfidfVectorizer(stop_words='english').get_stop_words())
# Add domain-specific stopwords discovered during topic review:
# custom_stopwords += ['reddit', 'post', 'comment', 'edit', 'deleted']

vectorizer = TfidfVectorizer(
    max_df=0.95,       # Ignore terms in >95% of docs
    min_df=2,          # Ignore terms in <2 docs
    max_features=2000, # Vocabulary cap
    stop_words=custom_stopwords,
    ngram_range=(1, 2) # Unigrams + bigrams for richer topics
)
tfidf_matrix = vectorizer.fit_transform(documents)
feature_names = vectorizer.get_feature_names_out()

Iterative stopword refinement: After an initial NMF run, inspect top terms per topic. If generic terms dominate multiple topics (e.g., "thing", "people", "really"), add them to custom_stopwords and refit.

Step 3: Select Number of Topics (k) via Coherence Sweep

Do NOT choose k arbitrarily. Sweep a range and score each.

from sklearn.decomposition import NMF
import numpy as np

def compute_coherence_umass(H, tfidf_matrix, top_n=10):
    """UMass coherence: co-occurrence of top words within documents.
    More negative = less coherent. Closer to 0 = better."""
    coherence_scores = []
    binary_matrix = (tfidf_matrix > 0).astype(int)
    doc_count = binary_matrix.shape[0]

    for topic_idx in range(H.shape[0]):
        top_word_indices = H[topic_idx].argsort()[-top_n:][::-1]
        topic_coherence = 0
        pair_count = 0
        for i in range(1, len(top_word_indices)):
            for j in range(i):
                w_i = top_word_indices[i]
                w_j = top_word_indices[j]
                co_doc_count = binary_matrix[:, w_i].multiply(
                    binary_matrix[:, w_j]
                ).sum()
                w_j_count = binary_matrix[:, w_j].sum()
                if w_j_count > 0:
                    topic_coherence += np.log(
                        (co_doc_count + 1) / w_j_count
                    )
                    pair_count += 1
        if pair_count > 0:
            coherence_scores.append(topic_coherence / pair_count)
    return np.mean(coherence_scores)

# Sweep k values
k_range = range(3, 21)
coherence_results = []

 k  k_range:
    model = NMF(n_components=k, init=, random_state=,
                max_iter=)
    W = model.fit_transform(tfidf_matrix)
    H = model.components_
    score = compute_coherence_umass(H, tfidf_matrix)
    reconstruction_err = model.reconstruction_err_
    coherence_results.append({
        : k, : score, : reconstruction_err
    })
    ()

Selecting k from results:

1. Plot coherence vs. k. Look for the "elbow" -- the point after which coherence stops improving meaningfully.
2. Cross-check with reconstruction error (should decrease monotonically; large jumps indicate instability).
3. Prefer the smaller k when two values produce similar coherence (parsimony).
4. If coherence is uniformly poor (all scores below -2.0 for UMass), the corpus may lack thematic structure. Report this finding.

Step 4: Fit Final NMF Model

k_chosen = 10  # Replace with value from Step 3

nmf_model = NMF(
    n_components=k_chosen,
    init='nndsvda',
    random_state=42,
    max_iter=500,
    alpha_W=0.1,
    alpha_H=0.1,
    l1_ratio=0.5
)
W = nmf_model.fit_transform(tfidf_matrix)  # Document-topic matrix
H = nmf_model.components_                   # Topic-term matrix

Step 5: Extract and Label Topics -- Human Review Required

def display_topics(H, feature_names, top_n=15):
    """Display top terms per topic for human labeling."""
    topics = []
    for topic_idx, topic_vec in enumerate(H):
        top_indices = topic_vec.argsort()[-top_n:][::-1]
        top_terms = [(feature_names[i], topic_vec[i]) for i in top_indices]
        topics.append(top_terms)
        terms_str = ', '.join(
            f"{term} ({weight:.3f})" for term, weight in top_terms
        )
        print(f"Topic {topic_idx}: {terms_str}")
    return topics

topics = display_topics(H, feature_names)

Labeling guidelines:

• Assign a short descriptive label based on the top 5-7 terms (e.g., "Gaming Hardware" not "Topic 3")
• If top terms are incoherent (no clear theme), label as "Incoherent -- Review" and note in report
• Do NOT force a label onto an incoherent topic. This is a signal, not a failure.
• Topics with very similar top terms may indicate k is too high -- consider reducing k

Step 6: Compute Document-Topic Distributions

import pandas as pd

# W matrix rows = documents, columns = topic weights
topic_labels = [f"Topic_{i}" for i in range(k_chosen)]
# Replace with human labels after Step 5:
# topic_labels = ["Gaming", "Tech", "Politics", ...]

doc_topic_df = pd.DataFrame(W, columns=topic_labels)
doc_topic_df['dominant_topic'] = doc_topic_df.idxmax(axis=1)
doc_topic_df['dominant_weight'] = doc_topic_df.max(axis=1)

# Documents with low dominant weight are multi-topic or off-topic
low_confidence = doc_topic_df[doc_topic_df['dominant_weight'] < 0.1]
print(f"Documents with weak topic assignment: {len(low_confidence)}/{len(doc_topic_df)}")

Interpreting weights:

• Dominant weight > 0.5: Strongly associated with one topic
• Dominant weight 0.2-0.5: Moderate association, likely multi-topic
• Dominant weight < 0.1: Weak assignment -- document may not fit any topic well

Step 7: Identify Cross-Topic Connections

# Topic co-occurrence: which topics frequently appear together in documents
topic_correlation = doc_topic_df[topic_labels].corr()

# Find documents that bridge multiple topics (high weight in 2+ topics)
def find_bridge_documents(W, threshold=0.15):
    """Documents with significant weight in multiple topics."""
    bridges = []
    for doc_idx, row in enumerate(W):
        significant = [(i, w) for i, w in enumerate(row) if w > threshold]
        if len(significant) >= 2:
            bridges.append({
                'doc_index': doc_idx,
                'topics': significant,
                'topic_count': len(significant)
            })
    return sorted(bridges, key=lambda x: x['topic_count'], reverse=True)

bridge_docs = find_bridge_documents(W)
print(f"Bridge documents (span 2+ topics): {len(bridge_docs)}/{len(W)}")

Cross-topic connections reveal the most interesting structural insights -- where seemingly separate themes intersect within the same documents.

Step 8: Compare with LDA (Optional Validation)

from sklearn.decomposition import LatentDirichletAllocation
from sklearn.feature_extraction.text import CountVectorizer

# LDA requires raw counts, not TF-IDF
count_vectorizer = CountVectorizer(
    max_df=0.95, min_df=2, max_features=2000,
    stop_words=custom_stopwords
)
count_matrix = count_vectorizer.fit_transform(documents)

lda_model = LatentDirichletAllocation(
    n_components=k_chosen, random_state=42, max_iter=50
)
lda_W = lda_model.fit_transform(count_matrix)

# Compare top terms -- do NMF and LDA agree on major themes?
# Agreement on core topics strengthens confidence in those themes.
# Divergence highlights topics that may be artifacts of one method.

Step 9: Write Report

Write all findings to docs/analysis/05-nmf-topic-modeling.md. See the report template below.

Report Output Template

The final report MUST be written to docs/analysis/05-nmf-topic-modeling.md with this structure:

# NMF Topic Modeling Analysis

## Methodology
- Corpus size: [N documents, average length]
- Preprocessing: [stopword list, ngram range, max_features]
- k selection: [range swept, coherence metric used, chosen k and why]
- NMF parameters: [init, solver, regularization]

## Coherence Scores
| k | Coherence (UMass) | Reconstruction Error |
|---|-------------------|---------------------|
| ... | ... | ... |

Selected k = [N] based on [elbow/plateau/domain judgment].

## Discovered Topics

### Topic 0: [Human Label]
**Top terms:** term1 (0.xxx), term2 (0.xxx), ...
**Coherence:** [topic-level score]
**Document count (dominant):** [N]
**Interpretation:** [1-2 sentence description]

[Repeat for each topic]

### Incoherent Topics
[List any topics that could not be meaningfully labeled, with top terms]

## Document-Topic Distributions
- Documents with strong single-topic assignment (>0.5): [N] ([%])
- Multi-topic documents (2+ topics >0.15): [N] ([%])
- Weakly assigned documents (<0.1 max weight): [N] ([%])

## Cross-Topic Connections
[Table or description of which topics co-occur, with example bridge documents]

## Topic Correlation Matrix
[Heatmap description or table of pairwise topic correlations]

## LDA Comparison (if performed)
[Which topics agreed across methods, which diverged]

## Limitations and Caveats
- [Corpus size limitations if any]
- [Incoherent topics that require further investigation]
- [Topics are statistical clusters, not definitive categories]
- [Results should be validated with domain expertise]

Common Mistakes

Mistake	Fix
Choosing k = 10 because "it feels right"	Sweep k range with coherence scoring. Use data, not intuition.
Skipping stopword iteration	Run NMF once, inspect topics, add noisy terms to stoplist, refit. Repeat.
Treating topics as definitive categories	Topics are statistical patterns. Always caveat that they require human validation.
Over-interpreting small topic weights	A document with weight 0.02 in a topic is NOT meaningfully associated. Use thresholds (>0.1 minimum).
Ignoring incoherent topics	Report them honestly. Incoherent topics signal k may be too high, or the corpus lacks structure in that region.
Using raw counts instead of TF-IDF for NMF	NMF works best with TF-IDF. Raw counts favor LDA. This is a fundamental method distinction.
Running on tiny corpus and trusting results	Below 50 docs, results are noise. Below 100, flag as exploratory.
Not comparing NMF with LDA	Cross-method agreement strengthens findings. Divergence flags fragile topics.
Labeling topics without reading example documents	Always read 5-10 documents from each topic to validate the label makes sense in context.

Red Flags -- Stop and Reassess

• All topics share the same dominant terms -- preprocessing failure (increase max_df, add stopwords)
• Coherence scores uniformly poor across all k values -- corpus may lack thematic structure
• Most documents have dominant weight < 0.1 -- topics do not capture document content
• Single topic absorbs >60% of documents -- either k is too low or corpus is thematically uniform
• Reconstruction error does NOT decrease as k increases -- model fitting problem (check data sparsity)

Boundaries

This skill SHOULD produce:

• 8-10 latent themes (adjustable based on coherence)
• Coherence scores for each k value tested
• Document-topic distribution matrix
• Cross-topic connection analysis
• Human-reviewed topic labels
• Written report at docs/analysis/05-nmf-topic-modeling.md

This skill should NOT:

• Claim topics represent "true" categories without human validation
• Use NMF results as sole basis for document classification
• Skip human review of topic interpretability
• Force labels onto incoherent topics
• Treat a single NMF run as definitive (iterate stopwords, compare with LDA)