Portfolio Backtesting: Multi-Symbol Strategies

Single-symbol backtests are easy but often misleading. A strategy that works great on SPY might collapse when applied to a portfolio of 50 stocks. Portfolio backtesting reveals the real behavior: capital constraints, correlation risk, and signal conflicts that don't exist in single-symbol tests.

The most common mistake is unlimited capital assumption. Your single-symbol backtest can enter every signal because there's always enough cash. In a real portfolio with fixed capital, you might get 20 buy signals on the same day and only have money for 5. Your backtest must model this explicitly — which 5 do you take?

Correlation matters enormously. Ten tech stocks might look like ten different bets, but they'll all crash together in a market panic. Real portfolio risk depends on how correlated your positions become in stress scenarios. A properly diversified backtest should include both correlated and uncorrelated instruments.

Signal ranking becomes the key decision. When multiple symbols trigger simultaneously, you need a ranking rule: highest expected return, best risk-reward, lowest volatility, highest liquidity? Different ranking rules produce dramatically different results. Test several to see which is most robust.

The most rigorous portfolio backtest simulates a real trading account: fixed starting capital, position size caps (e.g., max 5% per position), max total positions (e.g., 20 open at once), and cash allocation rules. This is far more work than single-symbol backtesting but gives results that actually resemble live trading.