Econometric Analysis: Looking at Flexibility in Models
Part of the Econometrics For Dummies Cheat Sheet
You may want to allow your econometric model to have some flexibility, because economic relationships are rarely linear. Many situations are subject to the "law" of diminishing marginal benefits and/or increasing marginal costs, which implies that the impact of the independent variables won't be constant (linear).
The precise functional form depends on your specific application, but the most common are as follows: