Since both L {\displaystyle L} and P {\displaystyle P} are fixed, and since the goal is to minimize w {\displaystyle w} , then the ratio ρ / σ {\displaystyle \rho /\sigma } should be minimized. By convention, however, the performance index is always a quantity which should be maximized. Therefore, the resulting equation is Performance index = P c r = σ / ρ {\displaystyle {\text{Performance index}}=P_{cr}=\sigma /\rho }