A single strategy, however good, will have drawdown periods. The purpose of portfolio construction is to combine multiple return streams — different strategies, instruments, or timeframes — so that one stream's drawdown is offset by another's gain, smoothing the combined equity curve without necessarily sacrificing return.
The Diversification Math
σ_portfolio² = w1²σ1² + w2²σ2² + 2·w1·w2·σ1·σ2·ρThe magic is in ρ, the correlation coefficient. If two strategies each have 15% annual volatility and are combined 50/50 with zero correlation, portfolio volatility is only 10.6% — not 15%. If correlation is 0.9 instead, portfolio volatility barely drops to 14.6%. Diversification benefit shrinks fast as correlation rises.
Position Sizing Approaches
| Method | Logic | Weakness |
|---|---|---|
| Equal weight | Allocate equal capital to each strategy/asset | High-vol assets dominate portfolio risk |
| Risk parity | Size inversely to volatility so each contributes equal risk | Can concentrate heavily in low-vol, illiquid assets |
| Kelly-based | Size proportional to edge and inversely to variance | Extremely sensitive to edge estimation errors |
| Mean-variance optimization | Solves for weights maximizing return per unit of risk | Unstable outputs with small changes in inputs |
During market crises, previously uncorrelated assets often move together as leveraged players deleverage across all positions simultaneously. A portfolio that looked well-diversified using 5 years of calm-market correlations can behave like a single concentrated bet during a crash.
Practical Steps for Retail Quants
- 1.Compute a rolling correlation matrix across all strategies/instruments, not just a single static one.
- 2.Stress-test the portfolio using historical crisis periods (2008, March 2020) where correlations spiked.
- 3.Cap total leverage and per-strategy allocation so no single correlated shock can breach your max drawdown limit.
- 4.Rebalance periodically — strategies that outperform tend to grow oversized relative to their intended risk budget.
The Bottom Line
Portfolio construction is where individually mediocre strategies can combine into something genuinely robust — and where individually great strategies can silently concentrate into unacceptable tail risk if correlation is ignored.