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# lec2 - Statistical Inference for FE Professor S Kou...

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Statistical Inference for FE Professor S. Kou, Department of IEOR, Columbia University Lecture 2. Properties of MLE and Estimation for Geometric Brownian Motion 1 Properties of MLE’s MLE is perhaps the most widely used estimation procedure, mainly because it leads to estimators with nice properties. Under certain regularity conditions, the MLE should have the following properties. (1) Consistency: ˆ θ θ in probability. (2) Asymptotic Normality ˆ θ N ( θ , I ( θ ) 1 ) , in distribution, where the Fisher information matrix I ( θ ) is given by I ( θ ) = E ( 2 log L ∂θ∂θ T ) . (3) It can shown the variance of the MLE is asymptotically smallest possible (i.e. achieve the Cramer-Rao lower bound). More precisely, no unbiased estimators can have smaller variance than that of the MLE, at least asymptotically. (4) Invariance. The MLE of C ( θ ) is given by C ( ˆ θ ) , if the function C ( · ) is continuously di ff erentiable. For example, if we want to estimate the Sharpe ratio ( μ r ) / σ , where r is the risk-free rate, we can simply use μ r ) / ˆ σ . 2 Estimation of Asymptotic Variances of MLE’s In the case of the MLE, we have the asymptotic variance is given by I ( θ ) 1 = Ã E " 2 log L ( θ ) ∂θ∂θ T #! 1 . However, since θ is unknown, we have to estimate I ( θ ) 1 . There are three ways to do that. (1) A natural estimator of the asymptotic variance I ( θ ) 1 is given by V A = I ( ˆ θ ) 1 . Of course, this approach depends on the assumption that I ( θ ) 1 can be computed easily or an analytical form of it is readily available. In general, computation of I ( θ ) 1 requires (possible highly dimensional) integration and inversion, with the highest possible dimension being the sample size. (2) If I ( θ ) 1 cannot be computed easily, then we can also estimate the asymptotic variance as V E ( ˆ θ ) where V E ( θ ) = Ã 2 log L ( θ ) ∂θ∂θ T ! 1 . 1

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In other words, we simply ignore the expectation. This works if the sample size is large enough and if V E ( θ ) can be written a sum of independent random variables, so that the law of large numbers applies. (3) Alternatively, we can use the BHHH estimator (Bernt, Hall, Hall, Hausman, 1974, Annals of Economic and Social Measurements). The idea of BHHH estimator is based on the observation that under suitable conditions V ar ( g i ) = E " 2 log f ( X i , θ ) ∂θ∂θ T # = I ( θ ) , E [ g i ] = 0 , (1) where g i = log f ( X i , θ ) ∂θ . In the appendix, we shall derive (1). Now suppose that the samples are independent (but not necessarily identically distributed). Then E " 2 log L ( θ ) ∂θ∂θ T # = n X i =1 E " 2 log f ( X i , θ ) ∂θ∂θ T # = n X i =1 V ar ( g i ) . The question becomes how to estimate V ar ( g i ) based on one observation X i . Since E [ g i ] = 0 , then a natural way to do this is to use g 2 i , resulting in the BHHH estimator V BHHH = " n X i =1 g i g T i # 1 , g i = log f ( X i , θ ) ∂θ , where n is the sample size.
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