\[ Vol = β_0 + (β_1*D) + ε \] \[ Vol = β_0 + (β_1*H) + ε \] \[ Vol = β_0 + (β_1*D) + (β_2*H) + ε \] \[ Vol = β_0 + (β_1*D) + (β_2*H) + (β_3*D*H) + ε \] \[ Vol = β_0 + (β_1*D^2) + β_2 (H^2) + ε \] \[ Vol = β_0 + (β_1*D) + (β_2*H)+ (β_3*D^2*H) + ε \] \[ Vol = β_0 + (β_1*D*H) + ε \] \[ Vol = β_0 + (β_1*D^2*H) + ε \] \[ Vol = β_0 + (β_1 D^2*H^2) + ε \] \[ lnVol = β_0 + (β_1*lnD) + ε \] \[ lnVol = β_0 + (β_1*lnH) + ε \] \[ lnVol = β_0 + (β_1*lnDH) + ε \] \[ lnVol = β_0 + (β_1*lnD^2H) + ε \] \[ lnVol = β_0 + (β_1*lnD^2H^2) + ε \] \[ lnVol = β_0 + (β_1*lnD) + (β_2*lnH) + (β_3*D^2H) + ε \] \[ lnVol = β_0 + (β_1*lnD) + (β_2*lnH) + ε \]