In the months leading up to last year's O-level examinations, Zachary Branson spent half of his eight hours of daily revision time ploughing through a stack of 10-year-series assessment books.
The former St Gabriel's Secondary School student did the questions in these books - which compile questions from past O-level exams and are updated every year - a few times.
Zachary, 17, who now attends Yishun Junior College, said: "If I got a question wrong, I would redo it until I got it right.
"Some of my classmates would memorise the answers for certain subjects, and they paid extra attention to repeated questions."
For four decades, students and teachers here have sworn by these books for N-, O- and A-level exam preparation, relying on repeated questions and model answers to gauge what examiners are likely to ask and look for.
Available at major bookstores from January each year, the books are usually sold out by August.
But in the last five to seven years, questions from past years tend not to be replicated, particularly for chemistry, economics, mathematics and physics, authors of 10-year-series books said.
The authors, mostly tutors with good track records, analyse the questions and supply model answers.
Questions for these exams are developed by the University of Cambridge International Examinations and Singapore's Ministry of Education. The ministry's Singapore Examinations and Assessment Board did not give reasons for the change.
It would only say that practice with 10-year-series books can familiarise students with the exam format, but they should not rely on them too much as they need to be able to apply the concepts they have learnt in new contexts.
The change has made exam preparation more difficult for some students. Some have turned to working on top schools' past-year papers, which are supposedly tougher.
Tutors have also had to use more creative teaching methods, instead of focusing on the 10-year-series books.
Economics tutor Anthony Fok, 31, author of an A-level economics 10-year-series book, noted that the questions, especially multiple-choice ones, used to be phrased exactly the same way, and the options were merely jumbled in different years.
Students could fare well by just memorising the answers without understanding them.
He said: "In recent years, the exam questions tend not to be repeated. There is no way of assuring a passing grade even if a student memorises all the questions and answers."
He now teaches students economic theories by relating these to real-world scenarios using newspaper and magazine articles.
Former chemist-turned-tutor Sean Chua, 36, who has been an author of O- and A-level chemistry 10-year-series books for six and three years respectively, said previously, between three and six questions would be repeated in an O-level chemistry exam and some teachers even told their students to memorise the model answers.
Examiners have recently introduced questions that cannot be answered by regurgitating from 10-year-series books, thereby testing students' understanding of the subjects, he said.
For instance, last-year's O-level chemistry paper included questions based on the A-level syllabus. Recent papers for maths and economics also included more scenario-based questions.
Students who memorise model answers "may have difficulties answering similar questions which are phrased differently", said chemistry tutor and A-level chemistry 10-year-series book author Jasmine Wong, 31.
To help her students, she uses simple experiments, news articles and everyday situations to teach concepts covered in the syllabus.
Raffles Junior College student Jackie Tan, 18, who will be taking his A-level exams this year, said: "It is a disadvantage now that the questions do not repeat themselves as much as before. The past-year questions give us a useful gauge to check if we are ready for the exams."
Housewife Angie Yeoh, 52, whose 16-year-old daughter is taking her O levels this year, said: "It is good that the questions allow students to apply what they've learnt instead of what they've memorised. Then they will be better prepared for real-life situations."