© 1994 by Biometrika Trust
MISCELLANEA |
A note on nonregular likelihood functions in heteroscedastic regression models
Department of Statistical Science, University College London Gower Street, London WC1E 6BT, U.K.
Received for publication 1 September 1993.
Revision received 1 March 1994.
| Abstract |
|---|
The existence of maximum likelihood estimates for a class of heteroscedastic regression models is considered. For a given dispersion function we show that, under a weak condition, the likelihood is singular at points corresponding to nonreplicated observations, causing unrestricted maximum likelihood estimation to break down, whilst for an alternative class of dispersion functions we obtain a much stronger linear independence condition for the likelihood to be unbounded.
Key Words: Mean-variance relationship Multimodality Normal-mixture Singularity