Walk Forward Leakage Auditor

Walk-Forward Leakage Auditor

Core Principle

Before trusting reported metric, state the temporal contract:

For a prediction made at <decision_time>, allowed inputs are <data sources> as known no later than <cutoff_time>. The target covers <future horizon>. Training may use rows ending at <train_end>, validation/calibration may use rows <validation_window>, and final reported performance comes only from <test_window>.

If the project cannot state this contract, the evaluation is not yet auditable.

Audit Workflow

1. Map the evaluation timeline.

   • Identify the prediction timestamp, label horizon, feature cutoff, train window, validation/calibration window, test/scoring window, embargo/gap, and retrain cadence.
   • Verify that every reported metric is computed only after the model, thresholds, transforms, and selection rules are frozen for that test window.

2. Trace data through the pipeline.

   • Follow raw data into features, labels, datasets, splits, fitted transforms, models, predictions, decisions, and metrics.
   • Verify that every fitted object is trained inside the fold/window where it is used.
   • Distinguish label construction from model features: label code may look forward, but label columns must be excluded from model inputs.

3. Inspect code and artifacts against the checklist below.

   • For each suspicious pattern, decide: leak, harmless by construction, or needs a test/guard.
   • Prefer concrete file/line evidence over broad claims.

4. Require a failable guard before trusting the result.

   • Add assertions for split ordering, no overlap, embargo, feature availability cutoff, fitted-transform scope, and comparable run controls.
   • Prefer a tiny synthetic fixture that fails if a future value leaks into a past prediction.
   • Rerun affected results after fixing leakage or faulty logic.

Split Logic Checklist

Check:

• Train rows end before validation/test rows begin.
• Embargo/gap is at least as long as the label horizon or any overlapping feature lookback that can leak target information.
• Fold boundaries are based on event/prediction time, not row order after arbitrary filtering.
• Same entity/event cannot appear in both train and test when that creates leakage.
• Hyperparameter, threshold, feature-selection, and architecture choices do not use final test metrics.

Red flags:

• train_test_split, KFold, StratifiedKFold, .sample(frac=...), or shuffle=True.
• Selecting "best" model/threshold by test Sharpe, test AUC, test return, or final leaderboard score.
• Reusing the same validation slice for early stopping, calibration, feature selection, and threshold selection without acknowledging optimism.

Feature Timing Checklist

Check:

• Rolling, expanding, rank, z-score, and aggregation features are lagged to the prediction cutoff.
• Cross-sectional features only use constituents and data known at the prediction time.
• Joins to fundamentals, events, news, status, or labels use release/effective timestamps, not file dates or period end dates.
• Imputation does not backfill from future observations.

Red flags:

• shift(-1) or negative periods in feature code.
• .rolling(...) or .expanding(...) used on a feature without a following lag/shift.
• bfill, fillna(method="bfill"), interpolation across the train/test boundary.
• merge_asof(..., direction="forward").
• Features named like future_*, next_*, target_*, label_*, outcome_*, realized_*, exit_*, or pnl_*.

Fitted Transform Scope

Every fitted object must be fit inside the training fold/window:

• Scalers and normalizers.
• Imputers.
• Encoders.
• PCA/UMAP/SVD.
• Feature selectors.
• Target encoders and count/frequency encoders.
• Oversampling/SMOTE/class balancing.
• Calibrators.
• Threshold selectors.

Red flags:

• fit_transform on the full dataset before split.
• Global means/stds/min/max/quantiles used to normalize the full timeline.
• Calibration trained on test predictions.

Label And Outcome Construction

Labels may use future data, but only inside label-generation artifacts that are excluded from model features. Check:

• Label columns are explicitly excluded by name or schema.
• Ambiguous/incomplete horizons are handled consistently.
• Rows near the dataset end are not scored when labels are unavailable unless running true live-mode inference.
• The target horizon cannot overlap the training window for the next fold without embargo.

Backtest And Simulation Logic

Check:

• Decision prices, fills, spreads, borrow availability, liquidity, and latency are available at decision time or modeled realistically.
• Position sizing uses only pre-trade account state.
• Trade selection thresholds are fixed from training/validation, not final test performance.
• Metrics include costs/slippage when the claimed result depends on tradability.

Comparable Experiment Checks

Fail a comparison when any of these differ unless the difference is the explicit treatment:

• Dataset version/hash/row count.
• Date range, cohort, universe, or entity membership.
• Target definition or horizon.
• Starting capital, budget, leverage, exposure, or cost model.
• Split policy, embargo, retrain cadence, and lookback.
• Metric definition, net/gross treatment, and aggregation level.

Review Standard

Lead with concrete findings and file/line references. For each issue, include:

• What leaks or is logically faulty.
• Why it changes evaluation results.
• The minimal fix or guard.
• Whether the result should be rerun.

If no leak is found, say which checks were performed and name the remaining unverified assumptions.

Common Fix Patterns

• Replace random splits with chronological splits or blocked/time-series CV.
• Fit scalers, imputers, encoders, feature selectors, calibration models, and threshold selectors inside each training window only.
• Add an embargo/gap when labels or feature windows can overlap the scoring horizon.
• Lag rolling/expanding features so the current prediction row cannot see its own outcome.
• Use backward/as-of joins with explicit timestamp tolerance for point-in-time data.
• Exclude labels, outcomes, future returns, post-event status fields, and model diagnostics from feature columns.
• Separate validation/calibration/tuning windows from final test windows.
• Save run metadata and fail comparisons when run controls differ.