Constant elasticity of variance model

In mathematical finance, the CEV or constant elasticity of variance model is a stochastic volatility model that attempts to capture stochastic volatility and the leverage effect. The model is widely used by practitioners in the financial industry, especially for modelling equities and commodities. It was developed by John Cox in 1975.

Dynamic

The CEV model describes a process which evolves according to the following stochastic differential equation:

:$\mathrmS_t = \mu S_t \mathrmt + \sigma S_t ^ \gamma \mathrmW_t$

in which ''S'' is the spot price, ''t'' is time, and ''μ'' is a parameter characterising the drift, ''σ'' and ''γ'' are other parameters, and ''W'' is a Brownian motion.
And so we have
:$\sigma(S_t, t)=\sigma S_t^$
The constant parameters $\sigma,\;\gamma$ satisfy the conditions $\sigma\geq 0,\;\gamma\geq 0$.

The parameter $\gamma$ controls the relationship between volatility and price, and is the central feature of the model. When $\gamma < 1$ we see an effect, commonly observed in equity markets, where the volatility of a stock increases as its price falls and the leverage ratio increases. Conversely, in commodity markets, we often observe $\gamma > 1$,Geman, H, and Shih, YF. 2009. "Modeling Commodity Prices under the CEV Model." The Journal of Alternative Investments 11 (3): 65–84. whereby the volatility of the price of a commodity tends to increase as its price increases and leverage ratio decreases. If we observe $\gamma = 0$ this model is considered the model which was proposed by Louis Bachelier in his PhD Thesis "The Theory of Speculation".

See also

*Volatility (finance)
*Stochastic volatility
* SABR volatility model
* CKLS process

References

External links

Asymptotic Approximations to CEV and SABR ModelsPrice and implied volatility under CEV model with closed formulas, Monte-Carlo and Finite Difference MethodPrice and implied volatility of European options in CEV Model
delamotte-b.fr

{{Stochastic processes

Options (finance)
Derivatives (finance)
Financial models