Nominal Outcome — Class Distribution Diagnostics & Primary Association Test

Table of Contents

• Scope Boundary
• Workflow
• Decision Tree
• Required Inputs
• Code Structure
• Reporting Standards
• Hypothesis Tests
• Example Dataset
• Method Status
• Minimal Smoke Test
• Cross-Skill Interface

Open-source skill.

Scope Boundary

Use this skill when:

• The outcome has 3 or more unordered classes and the first task is a baseline association test plus class-distribution check.
• A descriptive / inferential starting point is needed before multinomial or classifier-style modeling.

Do not use this skill when:

• The outcome has only 2 classes or has a meaningful order.
• The main need is repeated-measures, survival, or multilevel nominal modeling.

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
Test	Run Primary Test	Main Agent	workflow/step03-run-primary-test.md	Prior step output	PART 2-3 code blocks

Decision Tree

1. VALIDATE OUTCOME TYPE
   ├── Only 2 levels → redirect to vera-data-binary-reviewing
   ├── Ordered levels → redirect to vera-data-ordinal-reviewing
   └── 3+ unordered levels → proceed

2. PRIMARY ASSOCIATION TEST
   ├── Primary predictor is categorical → Chi-square + Cramér's V
   └── Primary predictor is continuous → One-way ANOVA (predictor ~ outcome) or Kruskal-Wallis

Required Inputs

Role	What to collect
Outcome (Y)	Variable name, what categories represent, confirm NO ordering
Number of levels	Confirm 3+ unordered categories
Primary predictor	Variable name, type (categorical or continuous)
Predictors	For recommendation block (not executed)
Covariates	For recommendation block (not executed)

Code Structure

PART 0: Setup & Data Loading
PART 1: Class Distribution Diagnostics → plot_01_class_distribution.png
PART 2: Primary Association Test       → plot_02_association_[var].png
PART 3: Recommendation Block           → text listing additional analyses available

Reporting Standards

1. p-values: "< .001" not "0.000"; exact to 3 decimals otherwise
2. Effect sizes: Cramer's V (Chi-square), eta-squared (ANOVA) — always alongside p
3. 95% CIs: always for effect sizes when available
4. Degrees of freedom: always with Chi-square and F statistics
5. Sample size: final analytic N
6. Decimal places: 2 for frequencies/proportions, 3 for p and effect sizes
7. Non-significance: "not statistically significant at alpha = .05" — never "no association"
8. Chi-square: report observed vs expected when informative
9. Always state which category is the reference (for later multinomial modeling)

Hypothesis Tests

Predictor Type	Primary Test	Confirmation
Categorical predictor	Chi-square + Cramer's V	Fisher's exact (if any cell < 5)
Continuous predictor	One-way ANOVA (predictor ~ outcome as factor)	Kruskal-Wallis

Example Dataset

R built-in iris: outcome = Species (setosa/versicolor/virginica, 3 unordered classes). Predictors: Sepal.Length, Sepal.Width, Petal.Length, Petal.Width. Python: from sklearn.datasets import load_iris (preferred — fully offline) or sm.datasets.get_rdataset("iris").

Method Status

Status	Methods
Implemented in this skill	Class-distribution diagnostics, chi-square / ANOVA-style association testing, rare-class warnings
Implemented downstream in vera-data-nominal-generating	Multinomial logistic regression, LDA, CART/RF/LightGBM, subgroup analysis, and IIA diagnostics
Out of scope in this open-source baseline	Binary, ordinal, nested-choice, or multilevel nominal workflows not named above

Minimal Smoke Test

• Smoke-test prompt: "Run vera-data-nominal-reviewing on iris, treating Species as the outcome and Petal.Length as the primary predictor. Produce the standard baseline artifacts."

Cross-Skill Interface

Output:
├── code_r      → .R script
├── code_python → .py script
├── figures/    → 2 PNGs (class distribution + association plot)
└── recommendations → text block (additional analyses available)

Next step: Invoke vera-data-nominal-generating from this skillset to run the full pipeline (additional tests, subgroup analysis, modeling, manuscript generation). See ../../CROSS-SKILL-INTERFACE.md for the shared handoff contract.