Statistics: Why and How

Spirit Island is a game of probabilities layered under strict mechanics. You can win by feel — most experienced players do — but at higher difficulty levels the feel-based approach hits a ceiling. Beyond that ceiling, counting starts paying.

This part gives you the mental models, the stat sources, and the reusable reasoning tools for playing the underlying math as a first-class part of your game.

What’s in this part

• Reading mindwanderer — how to interpret the community win-rate dashboard without misreading aggregate data. Includes Wilson-CI primer.
• Digital vs. Tabletop — why the Steam app’s win-rate distributions don’t map 1:1 to your physical table.
• Fear Card Expected Value — per-expansion Fear pool distribution + Monte Carlo per-game output.
• Event Deck Risk Profiles — balance + top spirit/invader-favoring events.
• Deck Expansion Impact — headline pool sizes + simulated per-game outcomes across all 5 decks × 6 expansion combos.
• Expansion Dilution Claims — variance analysis testing “B&C swingy, JE balances, NI dilutes” hypothesis.
• Blight Deck Severity — severity-proxy distribution.
• Token-Effect Dilution — per-token × per-deck × per-combo frequency.
• Aspect Power Deltas — how much each Aspect shifts its base spirit’s win rate.
• Major vs. Minor Draft Rates — when Minor-heavy beats Major-shopping.
• Self-Tracking Your Games — what to log, how to use si-post-game, the basics of a personal win-rate history.

The core statistical lenses

Four lenses run through every chapter:

1. Confidence, not point estimates. A win rate of 60% across 10 games isn’t 60% ± 0 — it’s closer to (26%, 88%) via Wilson 95%. Every rate here reports a CI where data supports one.
2. Conditional on expansion pool. The Minor deck at base-only is not the Minor deck at all-expansions. A “33% chance of drawing Moon-heavy” means different things in each pool.
3. Per-turn distributions, not per-game averages. “Averages 6 fear/game” hides whether that’s 6 evenly or 2+0+0+4+0. Shape is the signal.
4. Correlated vs. independent events. Fear-card draws depend on fear-pool threshold crossings; Event draws are independent per turn. The math differs.

Data provenance

• Wiki-parsed card data — data/decks/{minor,major,fear,event,blight}.json, parsed by scripts/wiki-fetch.py.
• Monte Carlo simulations — scripts/deck-simulate.py runs 5 000+ trials per expansion combo; outputs at data/references/deck-stats/sim_*.json.
• Community win-rate data — mindwanderer. Directional; subject to selection bias. Treated accordingly.