Now that we’ve discussed scaling in fair detail, we can fully understand the steps in which to calculate ATAR.
1) Firstly, your raw HSC mark used for scaling is calculated from your Raw Examination Mark and your raw moderated school assessment mark.
The HSC mark to be scaled is the average of your Raw Examination Mark (i.e. the mark you received in the external exam) and your raw moderated school assessment mark. The latter is determined through the process known as the HSC moderation process. (We recommend that you read more on moderation if you don’t know the process of how school assessment marks are treated)
This raw HSC mark will be scaled according to that particular subject’s scaling for that year. The scaled mark arrived at will be out of 50 per unit of study.
2) UAC takes the best 10 units (i.e. highest scaled marks for 10 units) including at least 2 units of English and adds up all scaled marks to form an Aggregate mark out of 500.
For example, if after scaling, your scaled marks were: 92/100 for English Advanced, 93/100 for Chemistry, 93/100 for Physics, 99/100 for Maths Extension 1, 99/100 for Maths Extension 2, your aggregate would be 92 + 93 + 93 + 99 + 99 = 476 / 500, (those marks correspond to top 1% in all subjects) and your ATAR would be a perfect 99.95. Note that if you do English Extension 1 or 2, any combination of 2 or more units of English can count to your ATAR. For example, 1 unit from English Advanced, 1 unit from English Extension 2.
3) The Aggregates of the entire cohort are compared and ATARs are assigned corresponding to the percentile of each Aggregate.
The percentile position of each Aggregate score relative to the entire cohort (including those who have left school at the end of year 10) corresponds to the ATAR given to that student, after rounding to the nearest 0.05 intervals. The final adjustment process effectively accounts for how the early school leavers would have done had they continued schooling and received a ATAR. Typically each year there are 21-23 students who achieve the perfect 99.95 ATAR, followed by around 41-43 students for each 0.05 increment thereafter.
So in simple terms, if your aggregate places you in the 99.85th percentile among your entire cohort, your ATAR would be 99.85.
WOW! What a mouthful that was. Actually the entire process is not that hard to understand in principle. What makes it difficult are the mathematical details, especially those on scaling and raw-mark conversion. But basically the scaling system operates according to statistical principles and is technically fair.