Comparison of Expected and Observed Fisher in Calculations for Estimates

X. Cao* and J. C. Spall*†

*JHU Department of Applied Mathematics and , Baltimore, MD; and †JHU Applied Physics Laboratory, Laurel, MD

PL is responsible for building mathematical models for many defense systems and other types of systems. These models often require statistical estimation of  from

data collected on the system. Maximum likelihood F–1(ˆ ) or H –1(ˆ ) , n n estimates (MLEs) are the most common type of statis- tical parameter estimates. Variance calculations and where F(ˆ ) is the average FIM at the MLE n confidence intervals for MLEs are commonly used (“expected” FIM) and H(ˆ ) is the average Hessian n in system identification and statistical inference for at the MLE (“observed” FIM). The question we the purpose of characterizing the uncertainty in the wish to answer is: which of the two approximations above, MLE. the inverse expected FIM or the inverse observed FIM, is Let ˆ be an MLE of u from a sample size of n. better for characterizing the uncertainty in the MLE? In n To accurately construct such confidence intervals, particular, the answer to this question applies to con- one typically needs to know the variance of the MLE, structing confidence intervals for the MLE. To answer the question, we find the variance estimate var(ˆ ). n T that minimizes the mean-squared error (MSE):

ˆ 2 min E[(n var( n ) – T) ] . APPROACH T Standard shows that the standard- In this work, T is constrained to be ized MLE is asymptotically normally distributed with a mean of zero and the variance equal to a function of F–1(ˆ ) or H –1(ˆ ) , n n . the Fisher information matrix (FIM) at the unknown parameter (see Section 13.3 in Ref. 1). Under certain assumptions, if we ignore a term of Two common approximations for the variance of magnitude o(n–1), F–1(ˆ ) tends to outperform H(ˆ ) n n the MLE are the inverse observed FIM (the same as the in estimating variance of normalized MLE. Hessian of the negative log-likelihood) and the inverse That is: expected FIM,2 both of which are evaluated at the MLE –1 2 –1 2 given sample data: E[(n var(ˆ ) – F (ˆ )) ] < E[(n var(ˆ ) – H (ˆ )) ]. A n n n n 294 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 28, NUMBER 3 (2010) FISHER INFORMATION IN VARIANCE CALCULATIONS FOR PARAMETER ESTIMATES

Table 1. MSE criterion for observed and expected FIM.

Example F–1(ˆ ) (Expected FIM) H–1(ˆ ) (Observed FIM) n n 1 0.02077 0.02691 2 0.007636 0.008238

• We have conducted numerical examples on the expected FIM tends to outperform a variance approxi- signal-plus-noise problem. mation based on the observed FIM under an MSE cri- terion. This result suggests that, under certain condi- • The estimation problem is the MLE for the variance of signal. tions, the inverse expected FIM is a better estimate for the variance of MLE when used in confidence interval • Table 1 shows the values of the MSE criterion for calculations. two distinct examples (observed and expected These results, however, are preliminary. Signifi- Fisher information represents candidate cant additional work will be required to better relate of the MLE variance estimate). the mathematical regularity conditions to practi- cal problems of interest at APL and elsewhere and to CONCLUSION extend the results to a multivariate parameter vector u. Furthermore, to the extent that the preferred The conclusion drawn from this work is that the approximation remains the inverse expected FIM, it expected Fisher information is better than the observed is expected that the of Spall3 Fisher information (i.e., it has a lower MSE), as predicted can be used to compute the expected FIM in com- by theory. plex problems of interest at APL and elsewhere. This The bottom line of this work is that, under reason- work is ongoing as part of the first author’s Ph.D. work able conditions, a variance approximation based on the at JHU.

For further information on the work reported here, see the reference below or contact [email protected].

1Spall, J. C., Introduction to Stochastic Search and Optimization: Estimation, Simulation, and Control, Wiley, Hoboken, NJ (2003). 2Efron, B., and Hinkley, D. V., “Assessing the Accuracy of the Maximum Likelihood : Observed Versus Expected Fisher Information,” 65(3), 457−482 (1978). 3Spall, J. C., “Monte Carlo Computation of the Fisher Information Matrix in Nonstandard Settings,” J. Comput. Graph. Stat. 14(4), 889−909 (2005).

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