## DEFINITION of 'Vasicek Interest Rate Model'

A method of modeling interest rate movement that describes the movement of an interest rate as a factor of market risk, time and equilibrium value that the rate tends to revert towards. This stochastic model is often used in the valuation of interest rate futures.

The Vasicek interest rate model values the instantaneous interest rate using the following equation:

dr_{t }= a(b-r_{t})dt +sdW_{t}

Where *Wt* is the random market risk (represented by the Wiener process)*t* represents time*a(b-r _{t})* represents the expected change in the interest rate at

*t*(drift factor)

*a*is the speed of reversion

*b*is the long-term level of the mean

*s*is the volatility at the time

## BREAKING DOWN 'Vasicek Interest Rate Model'

The Vasicek interest rate model states that the movement of interest rates is affected only by random market movements. In the absence of market shocks (i.e., when dW_{t }= 0) the interest rate remains constant (r_{t} = b). When r_{t} < b, the drift factor becomes positive, indicating that the interest rate will increase towards equilibrium.