In mathematical finance, the Cox–Ingersoll–Ross (CIR) model describes the evolution of interest rates. It is a type of “one factor model” (short rate model) as it describes interest rate movements as driven by only one source of market risk. The model can be used in the valuation of interest rate derivatives. It was introduced in 1985 by John C. Cox, Jonathan E. Ingersoll and Stephen A. Ross as an extension of the Vasicek model.
The model
The CIR model specifies that the instantaneous interest rate {\displaystyle r_{t}} follows the stochastic differential equation, also named the CIR Process:
 {\displaystyle dr_{t}=a(br_{t})\,dt+\sigma {\sqrt {r_{t}}}\,dW_{t}}
where {\displaystyle W_{t}} is a Wiener process (modelling the random market risk factor) and {\displaystyle a}, {\displaystyle b}, and {\displaystyle \sigma \,} are the parameters. The parameter {\displaystyle a} corresponds to the speed of adjustment to the mean {\displaystyle b}, and {\displaystyle \sigma \,} to volatility. The drift factor, {\displaystyle a(br_{t})}, is exactly the same as in the Vasicek model. It ensures mean reversion of the interest rate towards the long run value {\displaystyle b}, with speed of adjustment governed by the strictly positive parameter {\displaystyle a}.
The standard deviation factor, {\displaystyle \sigma {\sqrt {r_{t}}}}, avoids the possibility of negative interest rates for all positive values of {\displaystyle a} and {\displaystyle b}. An interest rate of zero is also precluded if the condition
 {\displaystyle 2ab\geq \sigma ^{2}\,}
is met. More generally, when the rate ({\displaystyle r_{t}}) is close to zero, the standard deviation ({\displaystyle \sigma {\sqrt {r_{t}}}}) also becomes very small, which dampens the effect of the random shock on the rate. Consequently, when the rate gets close to zero, its evolution becomes dominated by the drift factor, which pushes the rate upwards (towards equilibrium).
This process can be defined as a sum of squared Ornstein–Uhlenbeck process. The CIR is an ergodic process, and possesses a stationary distribution. The same process is used in the Heston model to model stochastic volatility.
Distribution
 Future distribution
 The distribution of future values of a CIR process can be computed in closed form:
 {\displaystyle r_{t+T}={\frac {Y}{2c}},}
 where {\displaystyle c={\frac {2a}{(1e^{aT})\sigma ^{2}}}}, and Y is a noncentral chisquared distribution with {\displaystyle {\frac {4ab}{\sigma ^{2}}}} degrees of freedom and noncentrality parameter {\displaystyle 2cr_{t}e^{aT}}. Formally the probability density function is:

 {\displaystyle f(r_{t+T};r_{t},a,b,\sigma )=c\,e^{uv}\left({\frac {v}{u}}\right)^{q/2}I_{q}(2{\sqrt {uv}}),}
 where {\displaystyle q={\frac {2ab}{\sigma ^{2}}}1}, {\displaystyle u=cr_{t}e^{aT}}, {\displaystyle v=cr_{t+T}}, and {\displaystyle I_{q}(2{\sqrt {uv}})} is a modified Bessel function of the first kind of order {\displaystyle q}.
 Asymptotic distribution
 Due to mean reversion, as time becomes large, the distribution of {\displaystyle r_{\infty }} will approach a gamma distribution with the probability density of:
 {\displaystyle f(r_{\infty };a,b,\sigma )={\frac {\beta ^{\alpha }}{\Gamma (\alpha )}}r_{\infty }^{\alpha 1}e^{\beta r_{\infty }},}
 where {\displaystyle \beta =2a/\sigma ^{2}} and {\displaystyle \alpha =2ab/\sigma ^{2}}.
show
Derivation of asymptotic distribution


Properties
 Mean reversion,
 Level dependent volatility ({\displaystyle \sigma {\sqrt {r_{t}}}}),
 For given positive {\displaystyle r_{0}} the process will never touch zero, if {\displaystyle 2ab\geq \sigma ^{2}}; otherwise it can occasionally touch the zero point,
 {\displaystyle \operatorname {E} [r_{t}\mid r_{0}]=r_{0}e^{at}+b(1e^{at})}, so long term mean is {\displaystyle b},
 {\displaystyle \operatorname {Var} [r_{t}\mid r_{0}]=r_{0}{\frac {\sigma ^{2}}{a}}(e^{at}e^{2at})+{\frac {b\sigma ^{2}}{2a}}(1e^{at})^{2}.}
Calibration
 Ordinary least squares
 The continuous SDE can be discretized as follows

 {\displaystyle r_{t+\Delta t}r_{t}=a(br_{t})\,\Delta t+\sigma \,{\sqrt {r_{t}\Delta t}}\varepsilon _{t},}
 which is equivalent to

 {\displaystyle {\frac {r_{t+\Delta t}r_{t}}{{\sqrt {r}}_{t}}}={\frac {ab\Delta t}{{\sqrt {r}}_{t}}}a{\sqrt {r}}_{t}\Delta t+\sigma \,{\sqrt {\Delta t}}\varepsilon _{t},}
 provided {\displaystyle \varepsilon _{t}} is n.i.i.d. (0,1). This equation can be used for a linear regression.
 Martingale estimation
 Maximum likelihood
Simulation
Stochastic simulation of the CIR process can be achieved using two variants:
 Discretization
 Exact
Bond pricing
Under the noarbitrage assumption, a bond may be priced using this interest rate process. The bond price is exponential affine in the interest rate:
 {\displaystyle P(t,T)=A(t,T)\exp(B(t,T)r_{t})\!}
where
 {\displaystyle A(t,T)=\left({\frac {2h\exp((a+h)(Tt)/2)}{2h+(a+h)(\exp((Tt)h)1)}}\right)^{2ab/\sigma ^{2}}}
 {\displaystyle B(t,T)={\frac {2(\exp((Tt)h)1)}{2h+(a+h)(\exp((Tt)h)1)}}}
 {\displaystyle h={\sqrt {a^{2}+2\sigma ^{2}}}}
Extensions
A CIR process is a special case of a basic affine jump diffusion, which still permits a closedform expression for bond prices. Time varying functions replacing coefficients can be introduced in the model in order to make it consistent with a preassigned term structure of interest rates and possibly volatilities. The most general approach is in Maghsoodi (1996). A more tractable approach is in Brigo and Mercurio (2001b) where an external timedependent shift is added to the model for consistency with an input term structure of rates. A significant extension of the CIR model to the case of stochastic mean and stochastic volatility is given by Lin Chen (1996) and is known as Chen model. A more recent extension is the socalled CIR # by Orlando, Mininni and Bufalo (2018,^{[1]} 2019,^{[2]}^{[3]}).
See also
 Hull–White model
 Vasicek model
 Chen model
References
 ^ Orlando, Giuseppe; Mininni, Rosa Maria; Bufalo, Michele (2018). “A New Approach to CIR ShortTerm Rates Modelling”. New Methods in Fixed Income Modeling. Contributions to Management Science. Springer International Publishing: 35–43. doi:10.1007/9783319952857_2. ISBN 9783319952840.
 ^ Orlando, Giuseppe; Mininni, Rosa Maria; Bufalo, Michele (1 January 2019). “A new approach to forecast market interest rates through the CIR model”. Studies in Economics and Finance. aheadofprint (aheadofprint). doi:10.1108/SEF0320190116. ISSN 10867376.
 ^ Orlando, Giuseppe; Mininni, Rosa Maria; Bufalo, Michele (19 August 2019). “Interest rates calibration with a CIR model”. The Journal of Risk Finance. 20 (4): 370–387. doi:10.1108/JRF0520190080. ISSN 15265943.
Ofer Abarbanel is a 25 year securities lending broker and expert who has advised many Israeli regulators, among them the Israel Tax Authority, with respect to stock loans, repurchase agreements and credit derivatives. Founder and CEO of TBIL.co STATX Mutual Fund.