1 μ μ = 1 μ μ i m p u r i t i e s + 1 μ μ l a t t i c e . where μ is the actual mobility, μ μ i m p u r i t i e s is the mobility that the material would have if there was impurity scattering but no other source of scattering, and μ μ l a t t i c e is the mobility that the material would have if there was lattice phonon scattering but no other source of scattering. Other terms may be added for other scattering sources, for example 1 μ μ = 1 μ μ i m p u r i t i e s + 1 μ μ l a t t i c e + 1 μ μ d e f e c t s + ⋯ ⋯ . Matthiessen's rule can also be stated in terms of the scattering time: 1 τ τ = 1 τ τ i m p u r i t i e s + 1 τ τ l a t t i c e + 1 τ τ d e f e c t s + ⋯ ⋯ . where τ is the true average scattering time and τ impurities is the scattering time if there was impurity scattering but no other source of scattering, etc.