Topic Modeling

Topic Modeling for Survey and Experimental Text Data

Instructions

1. Model Selection

• Default to Structural Topic Models (STM) when analyzing text from surveys or experiments. STM incorporates document-level metadata — treatment conditions, respondent demographics, country — directly into estimation, allowing prevalence and content to vary with covariates (Roberts et al. 2014).
• Use standard LDA only when no document-level covariates are needed and the corpus is large enough for unsupervised discovery (Blei, Ng & Jordan 2003).
• Consider BERTopic when working with short texts where word co-occurrence is sparse, or when multilingual embedding-based similarity is required (Grootendorst 2022). BERTopic clusters document embeddings via HDBSCAN, which yields hard single-topic-per-document assignments (unlike STM/LDA mixed membership), c-TF-IDF topic words that can be unstable on small corpora, and no native covariate framework. Use it when embedding-based semantic similarity matters more than mixed-membership structure or covariate estimation.
• Recognize the distinction between topic categorization and attitude inference: topic models reveal what respondents discussed, not what they believe. If the goal is inferring latent attitudes rather than categorizing surface content, complement topic modeling with an embeddings-based scaling of contextually common words or a separate supervised classifier (Hobbs & Green 2025; see also the companion text-classification skill).

2. Preprocessing

• Make preprocessing decisions explicit and justify each choice. Preprocessing is not neutral — stemming, stopword removal, and term-frequency thresholds all affect which topics emerge (Denny & Spirling 2018).
• Lowercase all text. Remove punctuation and numbers unless they carry substantive meaning in the domain.
• Remove stopwords using a standard list, but inspect the list for domain-relevant terms that should be retained (e.g., "foreign" in immigration research).
• Apply stemming only after checking that it does not collapse substantively distinct terms. Compare results with and without stemming as a robustness check (Denny & Spirling 2018).
• Set a lower-frequency threshold to prune very rare terms, and express it as a fraction of documents rather than an absolute count so it scales with corpus size. For small open-ended corpora a fixed count of 2–5 documents is often appropriate; for larger corpora, pruning terms that appear in fewer than roughly 0.5%–1% of documents is a common starting point (Grimmer & Stewart 2013). In stm, use plotRemoved() to visualize how many documents and terms are dropped across candidate thresholds before committing to one, then pass it to prepDocuments(). Report the threshold chosen and the counts of terms and documents retained.
• For translated text, preprocess after translation. Note that translation artifacts (e.g., inconsistent phrasing across translators) may affect topic coherence — document any translation pipeline.

3. Model Specification

• Specify the prevalence formula to include theoretically relevant covariates. For survey experiments, include treatment conditions; for cross-national data, include country. Example: prevalence = ~ treatment + country (Roberts et al. 2014). For experimental applications, pre-register the prevalence-covariate hypotheses (see the companion pre-registration-writing skill) so that estimateEffect() outputs are confirmatory rather than exploratory by default (Nosek et al. 2018).
• Optionally specify a content formula when you expect the words associated with a topic (not just its prevalence) to vary by covariate — e.g., different framings of the same topic across countries or treatment arms. Content covariates parameterize word distributions via SAGE-style deviations from a baseline, which complicates direct comparison of β across groups; inspect group-specific vocabulary with sageLabels() and interpret with care (Roberts, Stewart & Tingley 2014; 2019).
• Use spectral initialization (init.type = "Spectral") for reproducibility. Spectral initialization is deterministic on a given machine/BLAS configuration, unlike random initialization which requires multiple runs; cross-machine numerical precision can still produce minor differences, so record the hardware/OS when sharing replication materials (Roberts, Stewart & Tingley 2019).
• Set a random seed and record it regardless of initialization method. Also record the stm package version and R session info for DA-RT-compliant replication.

4. Selecting the Number of Topics

• Do not rely on a single metric. Evaluate candidate models across a range of K values (e.g., K = 5 to 30) using multiple diagnostics: semantic coherence, exclusivity, held-out likelihood, and residuals (Roberts, Stewart & Tingley 2019). In stm, searchK() sweeps over K and returns all four diagnostics at once; selectModel() fits multiple initializations at a fixed K and returns the coherence-exclusivity frontier (useful when spectral initialization is not being used).
• Semantic coherence measures whether high-probability words within a topic co-occur in the corpus. Higher is better, but coherence alone favors very common topics (Mimno et al. 2011). When a topic scores poorly, inspect the failure mode: Mimno et al. describe chained (two concepts linked by a shared word), intruded (unrelated words mixed in), random (no coherent connections), and unbalanced (mixes very general and very specific terms) — the diagnosis shapes whether the fix is K, preprocessing, or covariate specification.
• Exclusivity measures whether high-probability words are distinctive to one topic. The stm package reports FREX, a weighted harmonic mean of frequency and exclusivity ranks (default weight ω = 0.7). The coherence-FREX frontier identifies models that balance both (Roberts, Stewart & Tingley 2019).
• Treat held-out likelihood as one input among several, not the deciding metric. Chang et al. (2009) show that predictive likelihood can be negatively correlated with human interpretability — topic models that fit held-out words better are not necessarily more semantically coherent. When coherence, FREX, and held-out likelihood disagree, prefer interpretability (Chang et al. 2009).
• After narrowing candidates statistically, read the top words and representative documents for each topic. The final K should be interpretable — topics should be substantively meaningful and distinguishable from one another (Chang et al. 2009).
• Report the range of K values considered, the diagnostics used, and the rationale for the final choice.

5. Interpretation and Validation

• For each topic, report the top 10-15 words by probability and by FREX (frequency-exclusivity weighted). FREX words are often more interpretable because they are both common within the topic and rare outside it (Roberts, Stewart & Tingley 2019).
• Extract and read 3-5 representative documents per topic using findThoughts() or equivalent. Do not interpret topics from word lists alone — the documents provide essential context.
• Assign short substantive labels to each topic based on both word lists and representative documents. Labels should be descriptive (e.g., "economic contribution concerns") not technical (e.g., "Topic 7").
• Estimate covariate effects on topic prevalence using estimateEffect(). For experimental data, test whether treatment conditions significantly shift which topics respondents discuss. For cross-national data, test whether topic prevalence differs by country. Plot these effects with confidence intervals (Roberts et al. 2014). When testing many topics against a treatment, report corrections for multiple comparisons (e.g., FDR) alongside uncorrected estimates.
• To guard against spurious treatment-on-topic effects, use permutationTest() (Roberts, Stewart & Tingley 2014; 2019). The function permutes the treatment label and refits the STM, returning a null distribution of topic-level treatment effects against which the observed effect is compared — essentially a randomization inference check for estimateEffect().
• For construct validation in survey experiments: check whether topics related to the intended construct (e.g., civic engagement) show higher prevalence among respondents exposed to relevant treatment conditions. A null prevalence effect is suggestive of a construct-validity problem but does not establish one — rule out alternative explanations first, including insufficient statistical power, a suboptimal K that splits or merges the construct topic, a content-covariate effect (the manipulation shifted how the topic was framed rather than whether it was discussed), and preprocessing choices that dropped construct-relevant vocabulary.

6. Robustness and Sensitivity

• Run the model at neighboring values of K (e.g., K-2, K+2) to check whether substantive conclusions are sensitive to topic count.
• Compare results with and without stemming as a preprocessing robustness check (Denny & Spirling 2018).
• If using STM, compare results with and without covariates to assess how much the covariate structure shapes the topic solution.
• For experimental data, verify that topic assignments are not driven by a small number of high-frequency terms. Inspect whether removing the single most common term in a topic changes its interpretation.

7. Reporting

• Report: preprocessing pipeline (stopwords, stemming, frequency thresholds, counts of documents and terms retained after prepDocuments), number of topics and selection rationale, initialization method, convergence diagnostics (max EM iterations and final variational lower bound), random seed, stm package version, and R session info. For the broader reporting checklist see the companion methods-reporting skill.
• Present a topic summary table with: topic number, label, top FREX words, prevalence (overall and by covariate group), and 1-2 example documents.
• When reporting covariate effects on prevalence, present coefficient estimates with confidence intervals. Visualize cross-group differences (e.g., side-by-side prevalence plots by country or treatment).
• Distinguish discovery from confirmation: topic modeling is exploratory. State explicitly which findings were anticipated and which emerged from the data. Treating every topic-level comparison as a hypothesis test without that distinction re-creates the researcher-degrees-of-freedom problem documented in the false-positive literature (Simmons, Nelson & Simonsohn 2011; Nosek et al. 2018). If using topics to validate constructs, frame findings in terms of whether the intended constructs appeared, not whether the model "confirmed" a hypothesis (Grimmer & Stewart 2013).
• Make replication materials available: analysis code, preprocessing decisions, and random seed. If the corpus cannot be shared, provide the topic-term matrix and document-topic proportions (Grimmer & Stewart 2013).

Quality Checks

• Model type justified (STM vs. LDA vs. BERTopic) given data structure and research question
• Preprocessing decisions documented and justified (stemming, stopwords, thresholds)
• Number of topics selected using multiple diagnostics (searchK / selectModel over coherence, FREX/exclusivity, held-out likelihood, residuals) plus substantive interpretability
• Held-out likelihood vs. coherence disagreement resolved in favor of interpretability (Chang et al. 2009)
• Covariates included in prevalence (and, where justified, content) formula match the experimental or observational design; sageLabels() inspected when a content covariate is used
• Each topic interpreted using both word lists (probability + FREX) and representative documents via findThoughts()
• Covariate effects on topic prevalence estimated with estimateEffect() and reported with confidence intervals; multiple-comparison adjustment reported when testing many topics
• For experimental applications, permutationTest() run against the treatment covariate
• Robustness checks performed (alternative K, preprocessing variants, topic correlations via topicCorr())
• Topics labeled descriptively based on content, not just numbered
• Discovery vs. confirmation framing made explicit; confirmatory topic-prevalence hypotheses pre-registered where feasible
• Random seed, initialization method, convergence diagnostics, and plotRemoved() / prepDocuments retention counts reported
• Replication materials (code, preprocessing pipeline, seed, stm version, R session info) available or planned

Example

A minimal STM workflow for a survey experiment in which df holds one row per respondent with columns open_ended_responses (text) and treatment_arm (factor). The four stages — (a) preprocess, (b) sweep K, (c) refit with multiple initializations, (d) estimate treatment effects — map directly onto §§2–5. The four diagnostic functions (searchK, selectModel, permutationTest, topicCorr) should all appear in the final analysis script.

# Minimal STM workflow: survey experiment with open-ended responses.
library(stm)
set.seed(20260418)

# (a) Preprocess and prune rare terms (see §2).
processed <- textProcessor(df$open_ended_responses, metadata = df)
plotRemoved(processed$documents, lower.thresh = seq(1, 20, by = 2))
prepped <- prepDocuments(processed$documents, processed$vocab,
                         processed$meta, lower.thresh = 5)

# (b) Sweep K over candidate values (coherence, exclusivity, held-out, residuals).
k_search <- searchK(prepped$documents, prepped$vocab,
                    K = c(5, 10, 15, 20),
                    prevalence = ~ treatment_arm,
                    data = prepped$meta, init.type = "Spectral")
plot(k_search)

# (c) At the chosen K, refit with multiple RANDOM initializations; pick off the
# frontier. Do not pass init.type = "Spectral" here: spectral init is
# deterministic, so all runs would be identical and the frontier degenerate.
k_chosen  <- 10
candidates <- selectModel(prepped$documents, prepped$vocab, K = k_chosen,
                          prevalence = ~ treatment_arm, data = prepped$meta,
                          runs = 20, init.type = "LDA")
plotModels(candidates)
fit <- candidates$runout[[1]]

# (d) Estimate treatment effects on topic prevalence (§5); then permutationTest / topicCorr.
effects <- estimateEffect(1:k_chosen ~ treatment_arm, fit,
                          metadata = prepped$meta, uncertainty = "Global")
summary(effects)