Retrospective reserve is usually meant as the expected accumulated value at a given time of past premimums less benefits. With reference to multistate models, hoth the state of the policy at the time of valuation and its past story must be considered in calculating such expected value. Since it is unlikely that the retrospective reserve can be evaluated for any possible path of the risk, different definitious can be given according to the hypotheses on its story.
In this paper we firstly examine the main definitions of retrospective reserve given in the framework of multistate models. Then, we formulate a new definition which generalizes that given in conventional life insurance mathematics. Comparisons among the various definitions are proposed in some examples which, inter alias, show the different behaviour and magnitude of such reserves.