Therefore, the assumption that the force of mortality follows Gompertz's law is the necessary and sufficient criterion to guarantee that every joint-life status 'survival pattern can be replaced by a single-life status' in the group. 本文的结果,说明了生存群体中的联合状态的个体的生存分布可由生存群体中单生命个体的生存分布来代替的充要条件是,生存群服从Gompertz死亡力。