As mentioned in H2 Maths Distinction Rate (Percentage of As), the national distinction rate for H2 Math is usually around 50%. That is, 50% of all students will get ‘A’ grade for H2 Math. (Note that this statistic is heavily skewed by the top tier JCs where close to 80%-90% of all students get ‘A’ for H2 Math.)
According to numerous online sources, such as Reddit, this year’s (2018) H2 Math paper was significantly harder than previous years. It featured many “out-of-the-box” questions that are not found in the Ten-Year-Series. The “bug” question featuring a “bug” walking in a zig-zag fashion and falling into a “blackhole” stumped many students. Certain topics, such as PnC (Permutations and Combinations) are totally left out, prompting students to say that it is a case of “PnC —> “high investment Low returns “ More like “high investment NO returns”. (Update: The “bug” question is a disguised PnC question.)
Despite all these, there are still many students who honestly think that the paper was doable, or even easy. Probably these are the students who have practiced Prelim papers of top schools like Hwa Chong or Raffles, and hence are used to such tough questions.
The usual consensus is that a high 70s mark (>75) is sufficient to get A for H2 Math. Low 70s (70-74) is quite risky and may either be ‘B’ or ‘A’ grade depending on the difficulty of the paper.
Someone actually created a poll (https://strawpoll.com/zh7r7xh7) for the H2 Math 2018 scores. It is quite possible to predict your own score accurately since the answers will be circulating online soon after the exam. Based on the poll, ignoring the typo error that is “75-89” is meant to be “75-79”, we can see that the cut-off for ‘A’ grade for 2018 should be in the range 70-74. This is based on the hypothesis that the 50% distinction rate holds.
The bellcurve for A levels is apparently set by SEAB (Singapore Examinations and Assessment Board), while Cambridge does the marking. According to an “old examiner report“, Cambridge likened grading to swimming a lap. They said that a candidate who could ‘compete a lap’ would be given due credit regardless of the number of people who also completed the lap. After Cambridge finishes marking and gives the numerical score, the bell curving will be processed by SEAB to determine the final grade, that is, ‘A’, ‘B’, ‘C,’ and so on. This is probably necessary to ensure a “consistent” grade distribution so that university admissions can go smoothly, and that grades from different years are roughly comparable.