Why the same percentage doesn't always mean the same grade
Students often assume there's a fixed rule — "90% is always a grade 9", "40% is always a C" — and then panic when a mock or a past paper's published boundary doesn't match that assumption. There isn't a fixed rule. Grade boundaries are set after a paper is sat, based on how every student who took it actually performed, not decided in advance.
"Comparable outcomes" — the principle exam boards use
UK exam boards set boundaries using a principle called comparable outcomes: the goal is that a student of a given ability should get roughly the same grade regardless of which year they happened to sit the exam, even if that year's paper turned out to be a bit harder or easier than usual. If a paper is judged to have been more difficult than previous years, the raw-mark boundary for each grade is lowered to compensate; if it was easier, the boundary rises. This is also why a harder paper isn't actually bad news in itself — the boundary moves to account for it.
Who sets boundaries, and when
Senior examiners and statisticians at each exam board review a sample of real student scripts at different mark levels shortly after the exam series, compare overall national performance to previous years (and to other available evidence, like prior attainment data), and agree boundaries before results are issued. This is why boundaries for the same subject and board can shift by several marks from one year to the next — it's a deliberate response to that year's paper and that year's national cohort, not an error or inconsistency.
GCSE vs A-Level grading scales
| GCSE | A-Level |
|---|---|
| 9 (highest) down to 1 (lowest) | A* (highest) down to E (lowest passing grade) |
| Grade 4 = "standard pass" | No equivalent pass/fail line — E is the lowest reported grade |
| Grade 5 = "strong pass" | — |
| Set per paper, then combined across papers for the subject | Set per paper, then combined across papers for the subject |
Both scales are set using the same comparable-outcomes principle — only the labels and the number of grade points differ. (If you're studying IGCSE, see our IGCSE vs GCSE comparison for how its A*–G scale lines up against the GCSE 9–1 scale.)
What this means for your revision
Don't revise by chasing a fixed percentage target — chase understanding of what the mark scheme actually rewards at each level, since that's the thing that stays constant even when the raw-mark boundary moves. A mark scheme written in levels of response (see our guide to how mark schemes work) tells you what separates a Level 2 answer from a Level 3 one regardless of where the boundary lands that year — and that's a far more reliable thing to practise toward than a number that won't be confirmed until after you've already sat the exam.