  The distribution of the grid points at which a response function is observed
in longitudinal or functional data applications is often informative and not
independent of the response process. In this paper we introduce a covariation
model to estimate and make inferences about this interrelation, by treating the
data as replicated realizations of a marked point process. We derive maximum
likelihood estimators, the asymptotic distribution of the estimators, and study
the estimators' behavior by simulation. We apply the model to an online auction
data set and show that there is a strong correlation between bidding patterns
and price trajectories.
