We consider the Gaussian ARFIMA(j,d,l)
model, with spectral density
and an unknown mean
.
For this class of models, the n−1-normalized
information matrix of the full parameter vector, (μ,θ), is
asymptotically degenerate. To estimate θ, Dahlhaus (1989, Annals of Statistics 17,
1749–1766) suggested using the maximizer of the plug-in
log-likelihood,
,
where
is any n(1−2d)/2-consistent
estimator of μ. The resulting estimator is a plug-in maximum
likelihood estimator (PMLE). This estimator is asymptotically normal,
efficient, and consistent, but in finite samples it has some serious
drawbacks. Primarily, none of the Bartlett identities associated with
are satisfied for fixed n. Cheung and Diebold (1994, Journal of Econometrics 62,
301–316) conducted a Monte Carlo simulation study and reported that
the bias of the PMLE is about 3–4 times the bias of the regular
maximum likelihood estimator (MLE). In this paper, we derive asymptotic
expansions for the PMLE and show that its second-order bias is
contaminated by an additional term, which does not exist in regular cases.
This term arises because of the failure of the first Bartlett identity to
hold and seems to explain Cheung and Diebold's simulated results. We
derive similar expansions for the Whittle MLE, which is another estimator
tacitly using the plug-in principle. An application to the
ARFIMA(0,d,0) shows that the additional bias terms are
considerable.Research on this topic
commenced during 2000–2002, while the author was visiting the Cowles
Foundation for Research in Economics at Yale University. The author is
most grateful to the Cowles Foundation for their generous hospitality and
to Donald Andrews and Peter Phillips for numerous helpful
comments.