Log risk ratio ($\ell =\log (p_1/p_0)$)

The logarithm of a risk ratio is zero at equal risks, negative when the first risk is smaller, and positive when it is larger. For independent binomial event counts $d_1,d_0$ from samples $n_1,n_0$, its usual delta-method variance estimate is $1/d_1-1/n_1+1/d_0-1/n_0$. This approximation requires nonzero counts; zero-event arms need another procedure. Log estimates are convenient for inverse-variance meta-analysis, and exponentiation returns to the risk-ratio scale.