The CEV model describes a process which evolves according to the following stochastic differential equation:
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.[2]
In terms of general notation for a local volatility model, written as
we can write the price return volatility as
The constant parameters satisfy the conditions .
The parameter controls the relationship between volatility and price, and is the central feature of the model. When 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.[3] Conversely, in commodity markets, we often observe ,[4][5] whereby the volatility of the price of a commodity tends to increase as its price increases and leverage ratio decreases. If we observe this model becomes a geometric Brownian motion as in the Black-Scholes model, whereas if this model becomes the model which was proposed by Louis Bachelier in his PhD Thesis "The Theory of Speculation".