Table 1.

Functional forms under comparison

	g(x ij;θ)	θ	Model constraints
Linear	β x ij	(β)	β≤0
Logarithmic	βlog(x ij+1)	(β)	β≤0
Power	β(x ij+1)−γ	(β,γ)	β≥0, γ>0, α≠0
Exponential	βexp(−γ x ij)	(β,γ)	β≥0, γ>0, α≠0