Grade boundaries are set at the end of the marking period, which means it's only once all the papers have been marked that the awarding body will set the boundaries.
Although exam boards endeavour to set exam papers to the same level of difficulty each year, there will naturally be small variations in the level of challenge on the papers.
As a result, grade boundaries change each year to reflect the difficulty of the paper. This ensures that the standards are maintained from one year to the next, and avoids grade inflation.
Raw marks and UMS marks
If an exam is modular, you may have two sorts of marks, a raw mark and a UMS mark. The UMS mark is a unified mark scale, and the raw mark is converted into a UMS mark and then used to set the grade boundary. However, in linear exams without coursework, only the raw mark is used.
The decision-making process for grade boundaries is called "awarding", and it is undertaken by a group of senior examiners who are experts in their subject. This is overseen by Ofqual.
During awarding, the examiners compare scripts from the current exam series to previous years, and also look at the data of the cohort’s performance at KS2 Sats, to judge how they might be expected to perform in this exam.
How do tiered papers work?
Some exams have higher and foundation tiers. For these, the grade boundaries are set in the same way as above, but also by using what Ofqual call "common questions", which are the same on both higher and foundation papers.
These allow those setting the grade boundaries to align standards between the tiers, because they are able to see how all students respond to the common questions, regardless of which tier they have sat.
This means that a student who achieves a grade four on the foundation tier would also achieve a grade four on the higher tier. There is more information on how that works here.
What about grade 9?
Grade 9 is sometimes called "the new A*", but it actually is an entirely new grade, and was created as an answer to the criticism that the A* was becoming too common, and meaningless for employers and further education institutions who were unable to identify the highest-performing students.
In the first year of the new GCSE examinations the grade 9 is set arithmetically, using an equation to ensure only the very top performing students achieve it. The Department for Education describes it as the "percentage of those achieving at least a grade 7 who will be awarded a grade 9 = 7% + 0.5 × (percentage of students awarded grade 7 and above)".
After the first year of awarding, the grade 9 standard is carried forward from year to year as for other grades, using an approach based on a mixture of statistics and examiner judgement.
There is more detail on how this is calculated and a worked example here.
In order to ensure consistency, for English language, English literature and maths, the setting of the grade 9 boundary will be informed by the predicted performance of the cohort using KS2 data.
The figure only includes 16-year-olds, so older candidates can achieve a grade 9 without jeopardising the chance for a 16-year-old sitting the exam for the first time to do so.