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ON INCONSISTENCY OF THE COMMON RATE DIFFERENCE

ESTIMATORS FROM SPARSE FOLLOW-UP DATA

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TOSIYA SATO

*Department of Epidemiology, School of Health Sciences,*

University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan
(Received January 4, 1991; revised October 2, 1991)

**Abstract.**
It has been shown that the unconditional maximum
likelihood
estimator of the common odds ratio, risk ratio and risk difference
parameters are
inconsistent in sparse statification. Under a Poisson sparse-data model,
the maximum
likelihood estimator for the rate difference, which is the difference of
the disease
incidence rates among the exposed and the unexposed, is also shown to be
biased. The
sparse-data asymptotic bias of the maximum likelihood estimator is evaluated
numerically and compared with that of the weighted least squares estimators.

*Key words and phrases*:
Bias, epidemiologic methods, follow-up
studies,
Poisson observations, sparse-data.

**Source**
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