  Quarkonium production has been considered as a tool to study the medium
formed in high energy nuclear collisions, assuming that the formation of a hot
and dense environment modifies the production pattern observed in elementary
collisions. The basic features measured there are the relative fractions of
hidden to open heavy flavor and the relative fractions of the different hidden
heavy flavor states. Hence the essential question is if and how these
quantities are modified in nuclear collisions. We show how the relevant data
must be calibrated, i.e., what reference has to be used, in order to determine
this in a model-independent way.
