Cohort as in the whole state, yes. But that's often going under the assumption that HSC marks are using in the scaling process. In practise it's going to be impossible to distinguish between a hard exam and a hard subject when it comes to scaling. The marks across subjects are all compared between/across each other and what not and in the end the scaling of a subject is pretty much determined by how hard the subject, as well as that particular years exam, was because of that.