It depends on your rank within the cohort (ie. in terms of your SAC marks, rank 1 = the guy with the highest overall marks)
To copy and paste my response from another thread about how this works:
It's for every subject, and what it basically means is that your final SAC mark isn't based on your raw SAC marks in school, but your ranking in your school for that subject (that is, if you had the overall highest SAC marks, you'd be ranked first).
The way I saw it explained (somewhere on this board) was:
Say you have 3 people (Alice, Bob and Chris). Let's say they make up a cohort.
- Alice gets 80 for her SAC mark, 75 for exam
- Bob gets 60 for his SAC mark, 90 for exam
- Chris gets 40 for his SAC mark, 40 for exam
Alice, as rank 1 in SACs, would get a SAC mark based upon the highest exam mark in her cohort (which may or may not be her). In this case, this is Bob's exam mark of 90 (note that she still gets her exam score of 75, it's only her SAC mark that's based on her SAC ranking). Similarly, Bob as rank 2 in SACs would have his SAC mark based on Alice's exam performance and so on.
This is why the strength of your cohort matters, and if you go to a "bad" school you want to be rank 1 in SACs so it's based upon your own exam performance (if you're able to get the highest in your school).
This basically means that with a stronger cohort, you have a larger margin for failure in terms of your SACs because even if you don't end up as rank 1, you'll still get a good SAC mark because your cohort does so well.
Your own school cohort for a subject doesn't affect scaling -- scaling is determined by the strength of the entire cohort of a subject (that is how well people who do say, Biology, perform in their other subjects) and calculated accordingly. The "entire cohort of a subject" refers to everybody in the state who's doing that subject.