Extra-dispersion (overdispersion or underdispersion) is a common phenomenon in practice when the variance of count data differs from that of a Poisson model. This can arise when the data come from different subpopulations or when the assumption of independence is violated. This paper develops a...

The importance of the dispersion parameter in counts occurring in toxicology, biology, clinical medicine, epidemiology, and other similar studies is well known. A couple of procedures for the construction of confidence intervals (CIs) of the dispersion parameter have been investigated, but...

Here we prove that the LR test for one sided hypotheses concerning the coefficient of variation in an inverse Gaussian family is the UMP invariant test under scale transformation. Some approximations to the CDF of the test statistic are investigated.