很简单,就是它的本意之一“对于”.
这里牵扯到省略,补充完整应该是：The observed opinion change is greater than (the observed opinion change is) for persons who only hear the speech or for persons who read a prepared speech with emphasis solely on execution and manner of delivery.
全句可译：
那些只听演说的人,或者那些读演说稿时只关注讲话技巧和方式的人,对他们而言,所观察到的意见变更没有这么大.
——【译苦思甜】团队成员荣誉回答