# Parameter Estimation of Brownian motions by Method of Moments

How to estimate the parameters of a geometric Brownian motion (GBM)? It seems rather simple but actually took me quite some time to solve it. The most intuitive way is by using the method of moments.

# Estimation of ABM

First let us consider a simpler case, an arithmetic Brownian motion (ABM). The evolution is given by

\[ dS = \mu dt + \sigma dW. \]

By integrating both sides over \((t,t+T]\) we have

\[ \Delta \equiv S(t+T) - S(t) = \left(\mu - \frac{\sigma^2}{2}\right) T + \sigma W(T) \]

which follows a normal distribution with mean \((\mu - \sigma^2/2)T\) and variance \(\sigma^2 T\). That is to say, given \(T\) and i.i.d. observations \(\\{\Delta_1,\Delta_2,\ldots,\Delta_n\\}\) for different \(t\) values^{[1]}, with sample mean

\[ \hat{\mu}_{\Delta} = \frac{\sum_{i=1}^n\Delta_i}{n}\overset{p}{\to}\left(\mu - \frac{\sigma^2}{2}\right)T \]

and modified sample variance

\[ \hat{\sigma}_{\Delta}^2 = \frac{\sum_{i=1}^n (\Delta_i - \hat{\mu}_{\Delta})^2}{n-1} \overset{p}{\to} \sigma^2 T, \]

we have unbiased estimator for \(\mu\)

\[ \hat{\mu} = \frac{2\hat{\mu}_{\Delta} + \hat{\sigma}_{\Delta}^2}{2T} \]

and for \(\sigma^2\) we have

\[ \hat{\sigma}^2 = \frac{\hat{\sigma}_{\Delta}^2}{T}. \]

Now we prove the consistency. First we consider the variance of \(\hat{\mu}_{\Delta}\)

\[ Var(\hat{\mu}_{\Delta}) = \frac{Var(\Delta_1)}{n} = \frac{\sigma^2 T}{n} \]

and the variance of \(\hat{\sigma}_{\Delta}^2\)

\[ Var(\hat{\sigma}_{\Delta}^2) = E(\hat{\sigma}_{\Delta}^4) - E(\hat{\sigma}_{\Delta}^2)^2 = \frac{n E[(\Delta_1-\hat{\mu}_{\Delta})^4] + n(n-1) E[(\Delta_1-\hat{\mu}_{\Delta})^2]^2}{(n-1)^2} - \sigma^4T^2 = \frac{2\sigma^4T^2}{n}. \]

The variance of \(\hat{\mu}\) is therefore given by

\[ Var(\hat{\mu}) = \frac{4Var(\hat{\mu}_{\Delta}) + Var(\hat{\sigma}_{\Delta}^2)}{4T^2} = \frac{\sigma^2 (2 + \sigma^2T)}{2nT} \]

and the variance of \(\hat{\sigma}^2\) is given by

\[ Var(\hat{\sigma}^2) = \frac{Var(\hat{\sigma}_{\Delta}^2)}{T^2} = \frac{2\sigma^4}{n}. \]

So the two estimators are also both consistent. It should be noticed that there exists certain "trade-off" between the efficiency of \(\hat{\mu}_{\Delta}\) and \(\hat{\sigma}_{\Delta}^2\) by varying the value of \(T\).

# Estimation of GBM

For a general GBM with drift \(\mu\) and diffusion \(\sigma\), we have PDE

\[ \frac{dS}{S} = \mu dt + \sigma dW, \]

so we can integrate^{[2]} the both sides within \((t,t+T]\) for any \(t\) and get

\[ \Delta \equiv \ln S(t+T) - \ln S(t) = \left(\mu - \frac{\sigma^2}{2}\right) T + \sigma W(T). \]

The rest derivation is exactly the same.

# Validation

Now we numerically validate this against monte Carlo simulation.

1 | import numpy as np |

Statistics | monte Carlo | Method of moment | P Value |
---|---|---|---|

E(mu_hat) | 1.994533e-03 | 2.000000e-03 | 0.222191 |

Var(mu_hat) | 4.010866e-07 | 3.924000e-07 | - |

E(sigma2_hat) | 3.596733e-03 | 3.600000e-03 | 0.201573 |

Var(sigma2_hat) | 1.308537e-07 | 1.296000e-07 | - |

Now we may safely apply this estimation in application.

- 1.To assure that \(\{\Delta_i\}\) are i.i.d., we need time slots \((t,t+T]\) consecutive and have no overlay. Furthermore, in order to achieve the most efficient estimators for a given \(T\), it is clear that we opt for end-to-end slots over the total timespan. ↩︎
- 2.Although the LHS looks related to \(\ln(S)\), it's actually not. We need to use Itō calculus to derive this stochastic integral. ↩︎