Apa itu Roll’s estimator?

Roll’s estimator (Roll, 1984) of the effective bid–ask spread is $2\sqrt{-\gamma_1}$, with $\gamma_1$ the first autocovariance of trade-price changes: under pure bounce around an efficient price, trades are at mid $\pm c/2$ with independent signs, and $\gamma_1 = -c^2/4$.