Baseline Models

Baseline Models for Sports

Overview

If a fancy model cannot beat simple baselines under time-safe validation, it is not progress.

This skill defines the baseline ladder and standardizes matched, chronological comparison before ML.

Baselines are not disposable. Often they are the ship candidate.

When to Use This Skill

Use when:

• Starting a predictive sports project
• Auditing an ML result that looks shiny
• Building the first model you might actually keep
• User says “what’s the baseline?” or “does this beat home field?”
• Comparing form vs Elo vs constant on NFL/NBA/MLB

Do not skip this skill and jump to trees.

Installation

run_baselines.py requires pandas, NumPy, and scikit-learn; home_only_baseline.py requires pandas:

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python -m pip install pandas numpy scikit-learn

Parquet input also needs pyarrow or fastparquet. Both scripts expose --help before optional libraries are imported.

Baseline Ladder

Climb to ML only after Tier A–C (or A–D) exist under walk-forward evaluation.

The bundled fold helper implements binary Tier A and a numeric-feature Tier C logistic model. Tier B descriptive summaries have a separate helper; Elo, market, regression, and other ladder entries are methodology that require their own matched prediction artifacts.

Read references/baseline_ladder.md when selecting the minimum credible ladder and promotion rule. Read references/interpretation.md when deciding whether a candidate's delta is stable and practically meaningful.

Workflow

1. Define target + primary metric.
2. Build legal features (feature-rules).
3. Create walk-forward folds (validation-design).
4. Fit Tier A constant baseline each fold.
5. Fit Tier C logistic/linear baseline each fold.
6. Optional: rating-difference or market-reference baseline.
7. Compare per-fold and mean metrics.
8. Only then try ML (predictive-modeling).
9. Keep the simplest model that wins.
10. Log the experiment.

Run Baselines

Work from a user-owned table and use identical rows, folds, target, and scoring rules for every candidate.

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python /path/to/baseline-models/scripts/run_baselines.py \
  --input games.csv --target won --split-col season \
  --features is_home,rating_diff,rest_diff \
  --id-cols game_id,team --out baseline-folds.json \
  --predictions-out baseline-predictions.csv

python /path/to/baseline-models/scripts/home_only_baseline.py \
  --input games.csv --target won --home-col is_home

run_baselines.py evaluates a training-rate constant and regularized logistic baseline on expanding chronological folds. Its JSON artifact records the modeling contract, validation design, model names, per-fold metrics, means, and complete-case row accounting. It performs no imputation: rows missing target, split, or any named feature are excluded from both candidates and counted.

--min-train-groups default is 2. With seasons [2022, 2023, 2024], only 2024 is tested (train on 2022+2023). Use --min-train-groups 1 to also test 2023. The helper prints the planned test folds on stderr before scoring.

The prediction table writes identifiers, split/fold, y_true, constant_probability, logistic_probability, and portable p_pred (alias of the logistic probability for handoff).

Direct handoff to calibration:

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python /path/to/calibration-check/scripts/calibration_report.py \
  --input baseline-predictions.csv --target y_true --probability p_pred \
  --group-col season --out calibration.json

The home-only helper reports group rates and a continuity-corrected odds ratio with a Wald interval. Treat it as pooled descriptive inference unless it is embedded in a chronological evaluation; it does not by itself establish predictive generalization.

Tier Details

Tier A — Constant rate

The constant baseline predicts the training-fold mean of the target for every test row. On balanced team-game panels this is near 0.5 and log-loss near ~0.693.

Tier B — Home only

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python /path/to/baseline-models/scripts/home_only_baseline.py \
  --input games.csv --target won --home-col is_home

Tier C — Form logistic

Uses a small declared set of legal pre-event form differentials.

Tier D — Rating differential

Use a pre-event rating-difference column supplied in the modeling table and evaluate it on the same rows and folds as the other baselines.

What “Good” Looks Like

• Logistic log-loss < constant on most folds
• Coefficients point the right way (home effect positive in log-odds)
• Gains are not from one freak season only
• If trees barely beat logistic, prefer logistic

Decision table

Hard Constraints

1. No predictive project without Tier A.
2. Walk-forward only for claims of generalization.
3. Do not hide folds where baseline wins.
4. Simplest winning model is the default ship candidate.
5. Cross-sport claims require sport-specific evaluation; do not assume transportability.

Anti-Patterns

• Jumping to GBM with no constant baseline
• Reporting accuracy without log-loss/MAE
• One-season hero baselines
• Calling home-rate on the full doubled panel “home advantage”
• Hiding folds where ML loses to logistic

Reporting Template

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Baseline report
Sport/target/T:
Features:
Validation: season walk-forward
Tier A mean metric:
Tier C mean metric:
Tier D mean metric (if any):
Per-season table:
Decision: promote to ML | keep logistic | debug features
Reproduce:

Output Contract

Done means:

• Tier A present
• At least one stronger baseline (B/C/D) present
• Walk-forward comparison table present
• Decision stated
• Repro commands present

Bundled Resources

references/

scripts/

Related Skills

Quick Command Card

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python /path/to/baseline-models/scripts/run_baselines.py \
  --input games.csv --target won --split-col season \
  --features is_home,rating_diff,rest_diff \
  --id-cols game_id,team --out baseline-folds.json \
  --predictions-out baseline-predictions.csv

python /path/to/baseline-models/scripts/home_only_baseline.py \
  --input games.csv --target won --home-col is_home