Lookahead Bias

The most common ML4T failure. A model uses future information during training - normalization with full-sample statistics, labels from future thresholds, standard k-fold CV on time series - producing a great backtest that fails immediately in production.

The Problem

Lookahead bias means using information that would not have been available at prediction time. It inflates backtest Sharpe and is the #1 reason strategies fail live. Common sources: full-sample normalization, future-based thresholds, train+test preprocessing, and shuffled CV on time series.

The Pattern

WRONG

from sklearn.preprocessing import StandardScaler

# Normalize features using full dataset statistics (future leak)
scaler = StandardScaler()
X_scaled = scaler.fit_transform(X)  # mean/std computed on ALL data including test

# Label threshold from full history (future leak)
threshold = returns.quantile(0.75)  # Uses future returns to set threshold
y = (forward_returns > threshold).astype(int)

# Shuffled k-fold on time series (future leak)
from sklearn.model_selection import KFold
cv = KFold(n_splits=5, shuffle=True)  # Training on 2024 data, testing on 2020

CORRECT

from sklearn.preprocessing import StandardScaler
from sklearn.pipeline import Pipeline

# Expanding-window normalization: only past data
X_normalized = (X - X.expanding().mean().shift(1)) / X.expanding().std().shift(1)

# Expanding threshold: only past returns
threshold = returns.expanding().quantile(0.75).shift(1)
y = (forward_returns > threshold).astype(int)

# Time-series CV that preserves order
from sklearn.model_selection import TimeSeriesSplit
cv = TimeSeriesSplit(n_splits=5)

Preprocessing Pipeline

from sklearn.pipeline import Pipeline
from sklearn.preprocessing import StandardScaler
from sklearn.linear_model import Ridge

# Pipeline ensures scaler is fit on train only, even inside CV
pipe = Pipeline([
    ("scaler", StandardScaler()),
    ("model", Ridge()),
])
pipe.fit(X_train, y_train)       # scaler sees only training data
pipe.predict(X_test)             # test data transformed with train stats

Detection

• In-sample and out-of-sample performance are suspiciously similar (gap < 5%)
• Sharpe ratio > 2.0 on daily data with a simple model
• Feature values change when you recompute with more recent data appended
• Performance degrades sharply when switching from k-fold to walk-forward CV
• Labels use close-to-close returns but execution is next-open - this 50-100 bps gap per trade is hidden lookahead

Guardrails

• All rolling/expanding statistics must use .shift(1) to exclude the current observation
• fit_transform() must never touch test data - use Pipeline or manual train/test splits
• Cross-validation must respect temporal order - no shuffle=True on time series
• Label thresholds must be computed from past data only (expanding window)
• Point-in-time fundamentals: use report dates, not period dates

Production Implementation

from ml4t.diagnostic.splitters import CombinatorialCV
from ml4t.engineer import create_dataset_builder

builder = create_dataset_builder(features, labels, dates=timestamps, scaler="standard")
cv = CombinatorialCV(n_groups=8, n_test_groups=2, embargo_pct=0.01)
fold = next(builder.split(cv))
X_train, y_train = fold.X_train, fold.y_train
X_test, y_test = fold.X_test, fold.y_test

Checklist

• All normalizations use expanding window with .shift(1), not full-sample stats
• fit_transform() only on training data (use Pipeline or explicit split)
• Time-series CV with purging and embargo (not shuffled k-fold)
• Labels do not use future thresholds or statistics
• Point-in-time data: report dates used, not period-end dates
• Suspicious results investigated: Sharpe > 2, IS/OOS gap < 5%