Saturday, September 25, 2010

HSC Scaling & Subject Selection: How Do You Calculate Your ATAR?


I recently got an ATAR estimate which I was a little shocked and disappointed with, I had expected much higher. 
Is it possible to raise this by the actual HSC, or should I take the estimate as pretty spot on?
Also, we get moderated marks on our reports, are these supposed to reflect what our internal marks might scale to in the HSC, or what our overall marks might be? The difference in moderated marks between the first and last at my school in most subjects was very small, does this mean anything?

A number of students have asked me very similar questions over the last couple of weeks. As there appears to be a relative amount of confusion about ATAR calculation, internal vs external marks, and scaling, I thought I would dedicate this post to hopefully providing some clarity on the whole process.

ATAR Calculations are NOT ACCURATE

My first point is that if you have currently inputted your marks into an ATAR calculator and have received an estimate, you should not take this as gospel.

Why? Well for a couple of reasons. First of all, the marks you have used are only your internal school marks. Your ATAR will be calculated from both your external marks (from your actual HSC Exams counting for 50%, and your internal school marks which counts for another 50%). Secondly, you need to consider that scaling will also play a part – your internal school marks will be scaled or what is known as moderated based on your school’s performance in the HSC, and your external HSC exam marks will also be converted into marks on a common scale.

Why does this occur?

Well for starters, each school will provide slightly different assessments throughout the year, and have different standards of marking. As a result, it is unfair to say that a mark of 90% at School A is necessarily better than of 80% at School B. The difference could simply be that School B was given a harder assessment task and/or were marked considerably harder.

How Your Internal Marks are Moderated

As a result, to fairly compare student’s internal marks, these marks are moderated based on how the school performs as a cohort for that particular subject in the external HSC exams. For example, if the School B does exceptionally well in the external exams, the students internal marks at School B will be moderated upwards based on each individual students internal rank. As a result, if the whole cohort in School B does very well, all students internal marks will be moderated or scaled, in order of your internal ranking. So if you came 1st in the grade, your marks would scale the most, if you came 2nd, your marks would scale the 2nd most. This scaling of your internal marks is independent of how you actual do in your external HSC exams. It is completely reliant on how your whole grade does. Therefore, schools with ‘large tails’, or groups of students that will do poorly and pull the grade down will often have lower scaling. This is also an advantage for students at selective schools who do not have the ‘tail’ factor. (Note that this explanation has been simplified. The maths in terms of how marks are moderated is a little more complex. Marks are actually moderated based on the distribution of school assessment marks, but the above illustration aims to create a general idea of how this occurs).

In terms of the above question – the school cannot actually give you moderated marks. This is because they do not actually know how your grade will perform in the external HSC Exam and how your internal marks will in fact be moderated. As a result, it is likely they have estimated these moderated marks and they will therefore not be that accurate (and should not be relied upon anyway to give an accurate ATAR estimate).

What Really Matters

So, a couple of points:

  • Your ATAR calculation using only your raw internal marks, cannot give you a true indication of your ATAR score. The ATAR estimates will not factor in the scaling/moderation of your internal marks. For example, when I was in Year 12, the top mark in Chemistry (internally) was 70%. If you were to plug this mark in your ATAR calculator it would not be very good at all!
  • Guess who came 1st, 2nd and 3rd in Chemistry in NSW that year? Students from my high school. There internal marks were scaled/moderated to 98%+, based on how the whole grade performed in the external HSC exams and their internal rankings. This goes to show that what counts is not your internal marks, but your internal rank, and how your grade performs in the external HSC exams.

What About Your External Marks?

So far we have only been looking at what happens to your internal marks. What about your external marks. As noted, your external marks count for 50%. This is important, as it means that if you perform well in the external HSC exams, you can significantly boost your overall mark. So if you currently have not performed as well as you would have like in your trials, don’t worry. You still have hope. If you perform well in your external HSC exams, you can improve your overall marks and ATAR considerably (after all it counts for 50%).

Why are external HSC marks scaled, and how are they scaled?

Scaling occurs of your external marks to allow for a comparison across subjects. An aligned mark (raw marks which are ‘aligned to a Band – for example, 92% would be Band 6) of 96% in Chemistry is not necessarily better than a 92% in Physics (due to difficultly level for example). To compare marks fairly across subjects, these marks are converted into a common scale, which is donw by converting the raw external mark into a scaled mark out of either 50, or 100 depending on the subject (for e.g. 3U Maths/English is a mark of out 50, while Advanced Maths/English is a mark out of 100).

Technically, each subject is scaled not based on the difficulty of the subject, but on how well the candidature who take the subject does relative to your whole cohort (assuming that your whole cohort did the subject). Ever wondered why English is compulsory? It is used a s a cohort benchmark, from which all relative candidature performances are compared.

What does all this mean in plain English? It means that if a candidature do better in one subject than in their other subject, the subject will be scaled better. To illustrate with an example:

If there are 4,500 students taking Biology, who also take Economics, and Advanced English, and these 4,500 students do better than the students who took Economics and Advanced English, then their Biology marks will be scaled. The better they do compared to other people in Economics and Advanced English will result in their Biology marks scaling more.

Confusing isn’t it! If instead the Biology students scored lower than other students taking English Advanced, this would mean that Biology would not scale as well.

Basically, to cut straight to the point, there generally is a correlation between the degree to which a subject scales, and the difficulty. This is because students who take difficult subjects like Physics, 4U Maths, 4U English etc, tend do better at all their other subjects (compared to their peers) resulting in these difficult subjects scaling well.

So, what is the impact of scaling of your HSC external marks?

While scaling is designed to compensate for subject difficulty and the intelligence of the students taking these subjects (and therefore should not really affect you), it does. Because some subjects scale well (4U Maths for example), it means that you can do not as well (in terms of your marks) relative to the candidature (other students taking the subject) and it will still scale better than another subject.

What does this mean? Basically it means that you could get a mark of 85% in 4U Maths, which lets say will place you in the top 20% of students taking the course. Based on the fact that 4U Maths has a high scaled mean (how much the subject will scale – 4U Maths has had the highest scale mean since 2001 of around 44-45), your mark could scale to 94%.

Compare this to another subject like Physics are example. Because Physics has a lower scaled mean (around 30) this means to experience the same amount of scaling and do as well as your mark in 4U Maths, you will need to be in the Top 5-10% of the Physics cohort, as opposed to the top 20% in 4U Maths.

So, this basically means that taking any subject you can do very well (even ones that traditionally are seen as scaling very poorly, like Geography, which I did for my HSC) as long as you do exceptionally well compared to the rest of the cohort! This means that you really should pick subjects you enjoy and will do well at because you will have a higher likelihood of performing better in these subjects relative to the cohort.

To summarize:
  • ATAR calculators cannot give you an accurate estimate of your ATAR because you internal marks have not been moderated/scaled, and you do not yet have your scaled external marks.
  • Your internal marks will be moderated based on your internal rank. This means ranking in your school, as opposed to marks is what matters.
  • Your external marks are scaled based on the performance of the candidature relative to their performance in other subjects.
  • Often there is a correlation between the difficulty of the subject and scaling.
  • You should pick subject that you enjoy, because this will enable you to perform better in the subject relative to your peers improving your scaling



  1. Omgsh, you just made my day !
    I've been quite worried about my subjects recently, pretty much because of the fact that they're "low-scaling" subjects (I'm taking 'lightweight' subjects, Music, Art, D&T), having the mindset that my ATAR will be lower compared to other people in my cohort who do chemistry, physics, etc 'heavyweight subjects'.
    So you just cleared my confused head and made me feel more hopeful about taking the HSC exams next year :)

    Thank you thank you !

  2. Thanks for the feedback guys! Glad to hear that it helped :)

  3. ranking in your school... does that mean its harder to get a higher score in a selective school cos everyone is smarter?
    eg. in selective im ranked 30th but in non-selec im ranked 2nd?
    im a little confusedd ? :S

  4. Hey Jess,

    Yes and no!

    Yes, it is harder to rank well in a selective school - BUT, in a selective school, the whole grade performs very very well, meaning that your internal marks get scaled very well.

    Does this make sense?

  5. thank you for replying :)
    so... would you say you would get around the same marks? if my whole grade does well, why does my score get scaled higher?

  6. oh don't worry about the second question, you already answered in the article :)

  7. heyyy i was just wondering could internal ranks be calculated? my school offers extremely accurate atar estimates if i could know my overall internal ranks ]=

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  9. I go to a country school, I get 1st, 2nd and 3rd with relatively good marks but most of my grade don't do that well (depending on the subject) ...does that mean my marks will be brought down? We also have pretty small classes, does that change anything or..

  10. scaling doesn't correlate with the difficulty of the subject. humanities are devalued compared to physics, chemistry etc which are apparently 'harder.' You have to do EXTREMELY well in society and culture or music for example for them to scale well and its unfair. humanities are debatably just as difficult if not harder. I'm at a selective school and in the top 10 for all my subjects but I know that once scaling occurs I will be greatly affected as I don't do maths or the sciences.

  11. been helping the community of year 12 kids in our local community at the study lounge.

    going to be sharing some of this with the kids tomorrow. Very insightful and thank you :)

  12. hi i do low scaling subjects like
    general maths
    standard english
    business studies

    is it hard to get band 6 in general maths and band 5 in standard english????
    will i be able to get 90+ atar if i am interested in subjects like general maths and business as i think i can perform well in them????

  13. please reply??? i realy need help