Four hedge ratios, four different constraints
Let qA be the number of long A shares, qB the number of short B shares, and h = qB / qA. Prices PA and PB are currency per share. Then long notional is qA × PA and short notional is qB × PB. A positive h describes opposing positions; a negative fitted h implies buying B alongside A in this convention. Do not discard the sign to force a long-short trade.
- Dollar neutrality is an entry-time constraint unless you rebalance. Rebalancing changes holdings and incurs trading costs.
- Beta A and beta B must use the same benchmark, return definition, frequency and training dates. A near-zero beta B makes the ratio unstable; beta B = 0 makes this formula undefined.
- A price regression coefficient is a share ratio in this setup. A regression on percentage returns estimates a return sensitivity, requiring notional-to-share conversion. A log-price coefficient is an elasticity, not a fixed share count.
- For futures, include the contract multiplier and currency conversion in each contract value. Margin posted is not the contract notional. These share formulas do not directly size options or nonlinear exposures.
| Method | Shares of B per share of A | What it matches |
|---|---|---|
| Equal shares | h = 1 | Share counts only; share prices and economic exposure can differ. |
| Equal dollars | h = PA / PB | Initial long and short notionals in a common currency. |
| Benchmark beta | h = (beta A / beta B) × (PA / PB) | Estimated benchmark exposure: qA × PA × beta A = qB × PB × beta B. |
| Price OLS with intercept | h = sum((B - mean B) × (A - mean A)) / sum((B - mean B)^2) | Least-squares residuals of A on B over the training sample. |