In the field of referring expression generation, while in the static domain both intrinsic and extrinsic evaluations have been considered, extrinsic evaluation in the dynamic domain, such as in a situated collaborative dialog, has not been discussed in depth. In a dynamic domain, a crucial problem is that referring expressions do not make sense without an appropriate preceding dialog context. It is unrealistic for an evaluation to simply show a human evaluator the whole period from the beginning of a dialog up to the time point at which a referring expression is used. Hence, to make evaluation feasible it is indispensable to determine an appropriate shorter context. In order to investigate the context necessary to understand a referring expression in a situated collaborative dialog, we carried out an experiment with 33 evaluators and a Japanese referring expression corpus. The results contribute to finding the proper contexts for extrinsic evalution in dynamic domains.