Systematic selection for applications requiring multiple criteria is more complex. For example, when the material should be both stiff and light, for a rod a combination of high Young's modulus and low density indicates the best material, whereas for a plate the cube root of stiffness divided by density E 3 / ρ {\displaystyle {\sqrt[{3}]{E}}/\rho } is the best indicator, since a plate's bending stiffness scales by its thickness cubed. Similarly, again considering both stiffness and lightness, for a rod that will be pulled in tension the specific modulus, or modulus divided by density E / ρ {\displaystyle E/\rho } should be considered, whereas for a beam that will be subject to bending, the material index E 2 / ρ {\displaystyle {\sqrt[{2}]{E}}/\rho } is the best indicator.