<P> More formally, the income elasticity of demand, ε d (\ displaystyle \ \ epsilon _ (d)), for a given Marshallian demand function Q (I, P →) (\ displaystyle Q (I, (\ vec (P)))) for a good is </P> <Dl> <Dd> ε d = ∂ Q ∂ I I Q (\ displaystyle \ epsilon _ (d) = (\ frac (\ partial Q) (\ partial I)) (\ frac (I) (Q))) </Dd> </Dl> <Dd> ε d = ∂ Q ∂ I I Q (\ displaystyle \ epsilon _ (d) = (\ frac (\ partial Q) (\ partial I)) (\ frac (I) (Q))) </Dd> <P> or alternatively: </P>

Income elasticity of demand for normal good is always