Backtest Cost Model

Once a strategy passes the feasibility screen, the backtest engine still needs explicit cost settings. If commission, slippage, and impact are left at optimistic defaults, the simulation is still fiction.

The Problem

Most mistakes at this stage are implementation mistakes: missing volume in the feed, flat slippage for every asset, or no participation cap on large orders. The result is a backtest that claims to include costs while still materially understating them.

The Pattern

WRONG

import numpy as np

# Zero-cost backtest - fiction
positions = compute_positions(signals)
gross_returns = positions * asset_returns
sharpe = gross_returns.mean() / gross_returns.std() * np.sqrt(252)  # overstated

CORRECT

import numpy as np

def net_returns_with_costs(
    weights: np.ndarray,     # target weight per BAR for ONE asset, not shares
    asset_returns: np.ndarray,
    prices: np.ndarray,
    adv_shares: np.ndarray,  # average daily volume in shares, per bar
    daily_vol: np.ndarray,   # daily return volatility, per bar
    nav: float,
    commission_bps: float = 1.0,
    spread_bps: float = 5.0,
    impact_coeff: float = 0.1,
) -> np.ndarray:
    """Net returns after commission, spread and impact, all fractions of NAV.

    One asset, arrays indexed by bar. For a panel, run this per asset and sum:
    np.diff over a time-by-asset array differences neighbouring assets, not bars.
    """
    gross = weights * asset_returns
    traded_w = np.abs(np.diff(weights, prepend=0.0))  # traded fraction of NAV

    # Fixed costs: commission + half-spread on the traded notional
    fixed_cost = traded_w * (commission_bps + spread_bps / 2) / 10_000

    # Impact eta*sigma*sqrt(Q/ADV) as written below: Q is in shares, so take
    # the weight change through NAV and price before comparing it to ADV.
    participation = np.where(adv_shares > 0, traded_w * nav / prices / adv_shares, 0.0)
    # No upper cap: clipping at 1.0 prices a 3x-ADV order like an ADV-sized one
    impact_pct = impact_coeff * daily_vol * np.sqrt(np.clip(participation, 0, None))
    impact = impact_pct * traded_w  # a price move costs only what you traded

    return gross - fixed_cost - impact


def estimate_capacity(gross_sharpe, turnover, cost_bps_per_turn):
    """Rough capacity: the AUM at which costs consume alpha to the threshold."""
    alpha_bps = gross_sharpe * 100 / np.sqrt(252)  # daily alpha in bps (approx)
    cost_drag = turnover * cost_bps_per_turn / 252
    return f"Gross alpha ~{alpha_bps:.1f} bps/day, cost drag ~{cost_drag:.1f} bps/day"

Cost Components

Component	Typical Range	Scales With
Commission	0.5 - 10 bps	Trade count
Spread	1 - 50 bps	Asset liquidity
Slippage	1 - 20 bps	Order urgency
Market impact	5 - 100+ bps	Order size / ADV
Financing	25 - 300+ bps/yr	Short positions, leverage

Impact model: $\text{impact} = \eta \cdot \sigma \cdot \sqrt{\frac{Q}{\text{ADV}}}$ where $Q$ is order size, $\sigma$ is daily volatility, $\eta$ is a calibration constant (typically 0.05-0.3).

Guardrails

• Impact grows with the square root of participation rate - doubling AUM does not double cost
• Use asset-class appropriate estimates: crypto spread is 5-50 bps, US large-cap is 1-3 bps
• Short-side strategies must include borrow fees and financing - these can dominate total costs
• Validate cost assumptions against actual fill data (TCA) when available

Production Implementation

ml4t-backtest provides composable cost models:

from ml4t.backtest import BacktestConfig, CommissionType, DataFeed, Engine
from ml4t.backtest.config import SlippageType
from ml4t.backtest.execution.impact import SquareRootImpact
from ml4t.backtest.execution.limits import VolumeParticipationLimit

config = BacktestConfig(
    commission_type=CommissionType.PERCENTAGE, commission_rate=0.001,  # 10 bps
    slippage_type=SlippageType.VOLUME_BASED, slippage_rate=0.001,
)
engine = Engine(
    DataFeed(prices_df=prices), strategy, config,
    market_impact_model=SquareRootImpact(volatility=0.02),
    execution_limits=VolumeParticipationLimit(max_participation=0.10),
)

Checklist

• Feed includes volume so impact and participation limits are meaningful
• Market impact modeled for order sizes > 1% ADV
• Cost assumptions match asset class (not a single number for everything)
• Zero-cost and cost-aware runs compared to quantify implementation drag
• TCA or broker fill data used to calibrate rates when available