Portfolio and Multi-Asset Backtesting

Testing a rule on one asset is only the start. Real portfolios hold many positions, share limited capital, face constraints and rebalance over time. A portfolio backtest simulates all of this together: which assets to hold, how much of each, when to rebalance and what it all costs. Factor strategies, sector rotation, trend following across futures markets and multi strategy portfolios all require portfolio level testing to give realistic results.

Key design choices#

A simple example#

Universe management#

• Use point in time constituents: index members on each historical date.
• Handle delistings with final returns, including bankruptcies, often near minus 100%.
• Apply liquidity filters using past volume, so the strategy does not hold untradable stocks.
• Track corporate actions for splits, dividends, mergers and spin offs. See Corporate Actions, Delistings and Rolls in Backtests.

Constraints#

Portfolio metrics#

Beyond returns and Sharpe ratio, portfolio backtests should report:

• Turnover and total costs.
• Exposures: market beta, sectors, factors. See Factor Models.
• Concentration: largest positions and effective number of holdings.
• Drawdowns and recovery times. See Maximum Drawdown.
• Capacity: how returns change with assets under management. See Alpha Capacity and Crowding.
• Attribution: what drove returns. See P&L and Performance Attribution.

Common pitfalls#

Long short portfolios#

Long short backtests add complications: short positions need borrow availability and fees, short sale proceeds may earn interest, and gross exposure (long plus short) drives costs and risk. A market neutral portfolio should be checked for remaining beta, sector and factor exposures, since a portfolio that looks neutral by dollar amount can still carry hidden market risk. See Short Selling and Alpha and Beta.

Benchmarks#

Compare portfolio results with a relevant benchmark, such as an equal weight version of the same universe, not just a broad index. A strategy that beats the S&P 500 but trails an equal weighted small cap index may simply be capturing a size tilt rather than skill. See Size Factor.

Frequently asked questions#

What is a portfolio backtest?#

A simulation of a strategy across many assets at once, including selection, sizing, rebalancing, constraints and costs.

Why are portfolio backtests harder than single asset backtests?#

Because they require point in time universes, corporate actions, capital and margin limits, rebalancing logic and portfolio level costs.

What metrics should a portfolio backtest report?#

Returns, risk, drawdowns, turnover, costs, exposures to markets and factors, concentration and capacity.

Next, learn to handle splits, dividends and mergers in Corporate Actions, Delistings and Rolls in Backtests.