Drift Detection

A model trained on 2018-2022 data may silently fail when the 2023 distribution shifts. Systematic drift detection catches degradation before it becomes a drawdown.

The Problem

Financial distributions are non-stationary. Volatility regimes change, correlations spike during crises, and feature distributions shift as markets evolve. A model deployed without monitoring can trade on stale assumptions for months. The three drift types - data drift (input distributions change), concept drift (feature-target relationship changes), and prior drift (target distribution changes) - each require different detection strategies. By the time performance metrics visibly decay, the damage is already done.

The Pattern

WRONG

# Deploy model and check performance monthly - too late
model = train_model(X_train, y_train)
# ... 3 months later ...
print(f"Live Sharpe: {live_sharpe:.2f}")  # Already lost money

CORRECT

import numpy as np
from scipy.stats import ks_2samp

def calculate_psi(reference, current, n_bins=10):
    """Population Stability Index between two distributions."""
    breakpoints = np.percentile(reference, np.linspace(0, 100, n_bins + 1))
    # Open the outer bins. Percentile edges stop at the reference min and max,
    # so np.histogram silently drops every current observation outside that
    # range - exactly the tail shift PSI is supposed to catch.
    breakpoints[0], breakpoints[-1] = -np.inf, np.inf
    ref_pct = np.histogram(reference, bins=breakpoints)[0] / len(reference)
    cur_pct = np.histogram(current, bins=breakpoints)[0] / len(current)
    ref_pct = np.clip(ref_pct, 1e-4, None)
    cur_pct = np.clip(cur_pct, 1e-4, None)
    return float(np.sum((cur_pct - ref_pct) * np.log(cur_pct / ref_pct)))

# Monitor every feature at each rebalance
ref_data = X_train  # Training period as baseline
cur_data = X_live   # Most recent window

for col_idx, col_name in enumerate(feature_names):
    psi = calculate_psi(ref_data[:, col_idx], cur_data[:, col_idx])
    ks_stat, ks_pval = ks_2samp(ref_data[:, col_idx], cur_data[:, col_idx])
    flag = "DRIFT" if psi > 0.20 else "ok"
    print(f"{col_name:>25}: PSI={psi:.3f}, KS p={ks_pval:.3f} [{flag}]")

PSI Thresholds

PSI	Interpretation	Action
< 0.10	No significant drift	Continue
0.10 - 0.20	Moderate drift	Investigate, increase monitoring
> 0.20	Significant drift	Consider retraining

Detecting Concept Drift

Feature distributions can be stable while the feature-target relationship breaks. Track rolling IC to catch this:

from scipy.stats import spearmanr

def rolling_ic(predictions, actuals, window=63):
    """Rolling rank IC to detect concept drift."""
    ic_series = []
    for i in range(window, len(predictions)):
        ic, _ = spearmanr(predictions[i-window:i], actuals[i-window:i])
        ic_series.append(ic)
    return np.array(ic_series)

ic = rolling_ic(model_predictions, realized_returns)
baseline_ic = np.mean(ic[:252])  # First year as baseline
if np.mean(ic[-63:]) < baseline_ic * 0.5:
    print("ALERT: IC declined >50% from baseline")

Guardrails

• Baseline period must be representative and stable - do not use crisis periods as reference
• PSI and KS test catch different things: PSI is binned (better for tails), KS is continuous
• Some drift is normal in financial data - set thresholds based on historical drift rates, not arbitrary cutoffs
• Retrain triggers should be predefined: PSI > 0.20 → flag for review; rolling IC < 50% of baseline for 63+ days → trigger retrain pipeline
• Monitor prediction distribution too, not just features - a model can produce drifted outputs from stable inputs

Production Implementation

ml4t-diagnostic provides integrated drift monitoring:

from ml4t.diagnostic.api import cross_sectional_ic_series
from ml4t.diagnostic.evaluation.drift import analyze_drift

drift = analyze_drift(reference_df, current_df, methods=["psi", "wasserstein"])
ic_df = cross_sectional_ic_series(predictions, forward_returns, entity_col="symbol")
recent_ic = ic_df.tail(63)["ic"].mean()
baseline_ic = ic_df.head(252)["ic"].mean()
print(f"Baseline IC: {baseline_ic:.4f}, Recent IC: {recent_ic:.4f}")

Checklist

• PSI computed for all key features at each rebalance
• KS test run alongside PSI for continuous distribution comparison
• Rolling IC tracked for concept drift detection
• Alert thresholds predefined (not set after observing losses)
• Retrain protocol documented and triggered automatically on drift