Constant elasticity of substitution
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In economics, Constant elasticity of substitution (CES) is a property of some production functions and utility functions.
More precisely, it refers to a particular type of aggregator function which combines two or more types of consumption, or two or more types of productive inputs into an aggregate quantity. This aggregator function exhibits constant elasticity of substitution.
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[edit] CES production function
The CES production function is a type of production function that displays constant elasticity of substitution. In other words, the production technology has a constant percentage change in factor (e.g. labour and capital) proportions due to a percentage change in marginal rate of technical substitution. The two factor (Capital, Labor) CES production function introduced by Arrow, Chenery, Minhas, and Solow is:[1][2]
where
- Q = Output
- F = Factor productivity
- a = Share parameter
- K, L = Primary production factors (Capital and Labor)
- r =
- s =
= Elasticity of substitution.
As its name suggests, the CES production function exhibits constant elasticity of substitution between capital and labor. Leontief, linear and Cobb-Douglas production functions are special cases of CES production function. That is, in the limit as s approaches 1, we get the Cobb-Douglas function; as s approaches infinity we get the linear (perfect substitutes) function; and for s approaching 0, we get the Leontief (perfect complements) function. The general form of the CES production function is:
![Q = \left[\sum_{i=1}^n a_{i}^{\frac{1}{s}}X_{i}^{\frac{(s-1)}{s}}\ \right]^{\frac{s}{(s-1)}}](http://upload.wikimedia.org/math/2/5/0/25055e4ce1e47afad1e59c2cb29363bd.png)
where
- Q = Output
- F = Factor productivity
- a = Share parameter
- X = Production factors (i = 1,2...n)
- s = Elasticity of substitution.
Nested CES functions are commonly found in partial/general equilibrium models. Different nests (levels) allow for the introduction of the appropriate elasticity of substitution.
The CES is a neoclassical production function.
[edit] CES utility function
The same functional form arises as a utility function in consumer theory. For example, if there exist n types of consumption goods ci, then aggregate consumption C could be defined using the CES aggregator:
![C = \left[\sum_{i=1}^n a_{i}^{\frac{1}{s}}c_{i}^{\frac{(s-1)}{s}}\ \right]^{\frac{s}{(s-1)}}](http://upload.wikimedia.org/math/d/f/2/df2ac100e5c75bab054cdedc7806dcbf.png)
Here again, the coefficients ai are share parameters, and s is the elasticity of substitution. Therefore the consumption goods ci are perfect substitutes when s approaches infinity and perfect complements when s = 0. The CES aggregator is also sometimes called the Armington aggregator, which was discussed by Armington (1969).[3]
A CES utility function is one of the cases considered by Avinash Dixit and Joseph Stiglitz in their study of optimal product diversity in a context of monopolistic competition.[4]
[edit] References
- ^ Arrow, K. J.; Chenery, H. B.; Minhas, B. S.; Solow, R. M. (1961). "Capital-labor substitution and economic efficiency". Review of Economics and Statistics 43 (3): 225–250. http://www.jstor.org/pss/1927286.
- ^ Jorgensen, Dale W. (2000). Econometrics, vol. 1: Econometric Modelling of Producer Behavior. Cambridge, MA: MIT Press. p. 2. ISBN 0262100827.
- ^ Armington, P. S. (1969). "A theory of demand for products distinguished by place of production". IMF Staff Papers 16: 159–178.
- ^ Dixit, Avinash; Stiglitz, Joseph (1977). "Monopolistic Competition and Optimum Product Diversity". American Economic Review 67 (3): 297–308. http://www.jstor.org/pss/1831401.
