An $E$-Ancillarity Projection Property of Cox's Partial Score Function

Abstract

This paper shows that Cox's partial score function is the projection of the score function on the (locally) $E$-ancillary subspace for the nuisance parameter (Small and McLeish). This is done by adapting the concepts of (locally) $E$-ancillarity and (locally) $E$-sufficiency for inference functions (McLeish and Small) to an extended Cox's regression model, where the baseline function is allowed to be a predictable process.