L 3 Contingent immunization 问题 management, liability, because, active, higher

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原版书 Book 4 P94 Statement 2: When intereste rates fall, contingent immunization swithes to more active management because the dollar safety margin is higher.

请问老师这句话的成立是否有前提,还是总是准确? 还是能够详细解释一下,为什么利率下降, 应该更多采用active方式. 假设liability的duration小而进行或有免疫的asset的duration大, 那么利率下降, 是不是会使得dollar safety margin降低, 从而造成采用immunization而不是采用acitve management? 谢谢老师