My point, again, is that there is a logical difference between a within-group difference, within this half of the class, and a difference between groups, within--between this group and this group.

我的观点仍然是，在群体内部的差异与群体间的差异存在着逻辑上的区别，即在这半个班里的内部差异和在这个群体与那个群体之间的差异。