To allocate revision time well, share your available hours in proportion to the gap in each science, not in equal thirds. The gap is the marks you are currently losing, measured on questions you have marked honestly.
This skill matters in revision and transition decisions because Co-ordinated Sciences asks you to carry Biology, Chemistry and Physics at once. Check the current syllabus scope for 0654 on the Cambridge page before you plan.
Why do equal thirds fail?
Equal thirds assume all three sciences need the same help. They rarely do. A student who loses 20 marks in Chemistry and 8 in Biology gains more from moving hours towards Chemistry.
The opposite error also appears. Putting every hour into the weakest science lets the others slip, and slipped marks are expensive to win back.
How to build the split, step by step
- Mark a recent set of questions covering all three sciences, using the mark scheme.
- Record marks lost in each science. This is the gap.
- Add the three gaps to get the total gap.
- Divide each gap by the total, then multiply by your available hours.
- Round to whole hours and check the rounded numbers still add to your total.
Worked example
All data below are invented for practice. A student has 36 revision hours over six weeks. Marks lost in a marked set: Biology 8, Chemistry 20, Physics 12.
Total gap: 8 + 20 + 12 = 40.
Biology: 8/40 × 36 = 7.2 hours.
Chemistry: 20/40 × 36 = 18 hours.
Physics: 12/40 × 36 = 10.8 hours.
Rounded: 7 + 18 + 11 = 36 hours. The total still matches.
Then split each science’s hours across its weakest topics, so the plan names topics, not just subjects.
The mistake to watch for
A student with the same data writes “12 hours each” because it feels fair.
Mistaken plan: Biology 12, Chemistry 12, Physics 12.
Chemistry has half of all the lost marks but receives only a third of the time.
The correction is to let the gap decide. Use the equal split only when the three gaps really are equal.
Check yourself
All data are invented.
1. A student has 30 hours. Marks lost: Biology 10, Chemistry 10, Physics 20. Allocate the hours.
Show answer
Total gap = 40. Biology 10/40 × 30 = 7.5. Chemistry 7.5. Physics 20/40 × 30 = 15. Check: 7.5 + 7.5 + 15 = 30. Round as 7, 8 and 15, or keep half hours.
2. Marks lost are 6, 9 and 15 for 20 hours. Allocate to one decimal place.
Show answer
Total = 30. 6/30 × 20 = 4.0. 9/30 × 20 = 6.0. 15/30 × 20 = 10.0. Check: 4 + 6 + 10 = 20.
3. Why keep a small minimum for your strongest science?
Show answer
A subject left untouched loses fluency, so its gap grows. A short regular session keeps recall active without taking much time from the weaker sciences.
Where this leads next
Once the split is set, check that your notes match the right course in comparing a resource to the correct current syllabus. Practise the whole module with the revision and transition practice set, and use the scientific investigation critic when a gap is about evaluating experiments.
If you want someone to check your numbers and your topic order each fortnight, that is part of online one-to-one Co-ordinated Sciences tuition.