NLP Text Classification --- Data Diagnostics & Baseline Modeling

Table of Contents

• Scope Boundary
• Workflow
• Decision Tree
• Required Inputs
• Code Structure
• Reporting Standards
• Models Available
• Example Dataset
• Configuration Defaults
• Why These Defaults
• Minimal Smoke Test
• Cross-Skill Interface

Open-source skill.

Scope Boundary

Use this skill when:

• The task is supervised text classification with labeled examples.
• A lightweight, reproducible baseline is the right first checkpoint before deeper models.

Do not use this skill when:

• The task is generation, summarization, retrieval, topic modeling, or sequence labeling.
• Labels are absent, extremely sparse, or only available through prompt-based weak supervision.
• The core signal lives primarily in images, structured covariates, or temporal ordering rather than text.

Workflow

Read each step file in workflow/ before executing that step.

Step	Responsibility	Executor	Document	Input	Output
Collect	Collect Inputs	Main Agent	workflow/step01-collect-inputs.md	User input	Structured input summary
Diagnose	Check Distribution	Main Agent	workflow/step02-check-distribution.md	Prior step output	PART 1 code block
Baseline	Run Primary Test	Main Agent	workflow/step03-run-primary-test.md	Prior step output	PART 2-3 code blocks + T1 track artifacts

Decision Tree

1. CHECK CLASS BALANCE
   ├── Balanced (minority ≥ 10%) → standard methods
   └── Imbalanced (minority < 10%) → class weighting, note power limits

2. TEXT FEATURE STRATEGY
   ├── Vocab size < 50k → standard TF-IDF (max_features=10000)
   └── Vocab size ≥ 50k → sublinear TF-IDF with min_df=5
   # 10k features is the baseline default because it keeps sparse linear models
   # CPU-friendly while still covering common unigrams/bigrams in small-to-mid NLP datasets.
   # The 50k vocabulary gate is a heuristic: once the raw vocabulary gets very large,
   # sublinear TF-IDF and min_df filtering usually stabilize the baseline more than
   # throwing additional rare terms at Logistic Regression.

3. EXTRA FEATURES
   ├── Available → text + extra (augmented matrix)
   └── Not available → text-only (TF-IDF)

Required Inputs

Role	What to collect
Text column	Column name containing text data
Label column	Target variable, number of classes
Group column	For group-aware splitting (optional)
Extra features	Numeric columns for augmentation (optional)

Code Structure

PART 0: Setup & Data Loading
PART 1: Data Diagnostics          → plot_01_data_overview.png
PART 2: Baseline Classification   → plot_02_confusion_roc.png
PART 3: Recommendation Block      → text pointing to analysis workflow

Reporting Standards

1. Metrics: weighted F1 and macro AUC (OVR) — always with 95% bootstrapped CIs
2. Format: "F1 = 0.XXX, 95% CI [0.XXX, 0.XXX]"
3. AUC: "AUC = 0.XXX, 95% CI [0.XXX, 0.XXX]"
4. Decimal places: 3 for F1/AUC/p-values, 1 for percentages, 0 for counts
5. Proportions: report as percentages with 1 decimal
6. Sample size: report final analytic N (train/val/test split sizes)
7. Non-significance: "not statistically significant at alpha = .05" — never "no difference"
8. Bootstrapped CIs: 1000 iterations, 2.5th/97.5th percentiles
9. For imbalanced classes (minority < 10% of total, or per-class < 20%): report Precision–Recall AUC (PR-AUC / Average Precision) alongside F1 and ROC-AUC. PR-AUC is more informative than ROC-AUC when the positive class is rare because it does not reward true-negative inflation. Use sklearn.metrics.average_precision_score with macro averaging for multi-class. Frame in results as "For class imbalance, PR-AUC is the primary discrimination metric; ROC-AUC is reported for comparability with prior work."

Models Available

Status	Models
Implemented in this skill	Logistic Regression on TF-IDF features, optionally augmented with extra numeric metadata
Implemented downstream in vera-ai-nlp-generating	SVM, Random Forest, LightGBM, GRU, TextCNN, ALBERT
Not shipped in the open-source build	BERT, RoBERTa, DeBERTa, SetFit, prompt-only classifiers, GPT/Claude few-shot pipelines

Example Dataset

Any text classification dataset. Default: 20 Newsgroups (sklearn). Python: from sklearn.datasets import fetch_20newsgroups

Configuration Defaults

Pipeline constants live in config/default.json. Read it before generation — values there override anything in this SKILL.md. Key knobs:

• alpha (0.05), ci_level (0.95) — inference thresholds
• plot_dpi (300), plot_width, plot_height — figure dimensions
• bootstrap_iterations (1000) — bootstrap CIs for metrics
• split.{test_size, val_size_of_train, random_state} — data-split reproducibility
• tfidf.{max_features, ngram_range, sublinear_tf, min_df} — text featurization
• baseline_model.* — LogReg baseline hyperparameter grid
• class_balance_threshold (0.10) — minority-class threshold for rare-event warning

To override: create config/local.json; the runtime merges local over default.

Why These Defaults

• class_balance_threshold = 0.10 is a warning heuristic, not a theorem. Below about 10% minority prevalence, weighted F1 and ROC-AUC often look healthier than the actual positive-class retrieval story, so the skill promotes PR-AUC in that regime.
• bootstrap_iterations = 1000 is the default compromise between stable percentile intervals and a baseline run that still finishes comfortably on CPU.
• tfidf.max_features = 10000 keeps the baseline sparse-linear model reproducible and fast enough for smoke tests while still covering the dominant unigram/bigram signal in most benchmark-scale corpora.

Minimal Smoke Test

• Smoke-test prompt: "Run vera-ai-nlp-reviewing on a 4-class subset of fetch_20newsgroups, using the text as input and the target as label. Produce the standard T1 baseline artifacts."
• Suggested fixture: 200-1000 rows total, 50-250 rows per class, no extra features required.
• Expected pass condition: the run writes one runnable Python script, methods.md, results.md, tables/, references.bib, and the two baseline figures listed below.

Cross-Skill Interface

Output:
├── code_python      → .py script
├── methods_md       → methods.md baseline fragment
├── results_md       → results.md baseline results fragment
├── tables/          → Markdown/CSV tables for class balance + metrics
├── figures/         → 2 PNGs (class balance + confusion/ROC)
├── references_bib   → .bib with baseline/evaluation citations
└── recommendations  → text block (what analysis workflow produces)

When this skill is used as T1_baseline inside vera-ai-application-pipelining, the standardized track artifacts above are REQUIRED, not optional.