The score test can be inconsistent because-at the MLE under the null hypothesis-the observed information matrix generates negative variance estimates. The test can also be inconsistent if the expected likelihood equation has spurious roots.KEY WORDS: Maximum likelihood, score test inconsistent, observed information, multiple roots for likelihood equationTo appear in The American Statistician vol. 61 (2007) pp. 291-295
The short answerAfter a sketch of likelihood theory, this paper will answer the question in the title. In brief, suppose we use Rao's score test, normalized by observed information rather than expected information. Furthermore, we compute observed information atθ S , the parameter value maximizing the log likelihood over the null hypothesis. This is a restricted maximum. At a restricted maximum, observed information can generate negative variance estimates-which makes inconsistency possible.At the unrestricted maximum, observed information will typically be positive definite. Thus, if observed information is computed at the unrestricted MLE, consistency should be restored. The "estimated expected" information is also a good option, when it can be obtained in closed form. (Similar considerations apply to the Wald test.) However, the score test may have limited power if the "expected likelihood equation" has spurious roots: details are in section 8 below.The discussion provides some context for the example in Morgan, Palmer, and Ridout (2007), and may clarify some of the inferential issues. Tedious complications are avoided by requiring "suitable regularity conditions" throughout: in essence, densities are positive, smooth functions that decay rapidly at infinity, and with some degree of uniformity. Mathematical depths can be fathomed another day.David A. Freedman is Professor, Department of Statistics, University of California Berkeley, CA 94720-3860 (E-mail: freedman@stat.berkeley.edu). Peter Westfall (Texas Tech) made many helpful comments, as did Morgan, Palmer, and Ridout.