Structural-credit metric from Merton's model: DD = dfracln(V/D) + (μ_V - tfrac12σ_V^2)Tσ_V√(T), the number of standard deviations the asset value V is above the debt level D at horizon T. Risk-neutral default probability is N(-DD). KMV (now Moody's Analytics) commercialised the model, calibrating asset value and vol from equity price via the option-pricing identity E = V,N(d_1) - D,e^-rTN(d_2). The benchmark structural credit measure.