What CER stands for
- Claim — the answer to the question. What happens, what is the result, what is observed.
- Evidence — the specific observation, measurement, or data from the question that supports the claim.
- Reasoning — the underlying mechanism. Why the evidence leads to the claim. This is where topic-specific keywords earn marks.
Why mark schemes reward CER
Open-ended Science questions test reasoning, not memorisation. Mark schemes typically allocate marks per "step" in the explanation: 1 mark for the correct claim, 1 mark for citing evidence correctly, 1–2 marks for the mechanism in correct scientific terms. A student writing in narrative form often produces the right answer but in a way that hides the marks.
Worked example — Primary level
Question: "When a metal spoon is left in a hot drink, the spoon becomes hot. Explain why."
Narrative answer (often 1/2): "The spoon becomes hot because of the heat from the drink."
CER answer (2/2):
- Claim: The spoon becomes hot.
- Evidence: The drink is at a higher temperature than the spoon.
- Reasoning: Heat is transferred from the hotter drink to the cooler metal spoon, until both reach the same temperature.
Worked example — Sec 3/4 level
Question: "A wire is connected to a battery and a switch. When the switch is closed, the bulb lights up. Explain why."
CER answer:
- Claim: The bulb lights up when the switch is closed.
- Evidence: Closing the switch completes the circuit, providing a continuous conducting path.
- Reasoning: The battery provides a potential difference that drives current through the circuit. Current flowing through the bulb's filament heats it to incandescence due to its high resistance, causing it to emit light.
By-level differences in CER depth
- PSLE — short CER. 2–4 lines. Topic keyword vocabulary at upper-primary level.
- N-Level / Combined Science — moderate CER. 4–6 lines. Keywords expand to specific terminology (e.g. kinetic energy, equilibrium, photosynthesis).
- O-Level Pure subjects — full CER. 6–10 lines for higher-mark questions. Reasoning section is the longest. Subject-specific keyword discipline is non-negotiable.
Common CER mistakes
- Skipping evidence. Going straight from claim to reasoning loses a mark.
- Vague reasoning. "Because heat moves" instead of "because heat is transferred from regions of higher temperature to regions of lower temperature."
- Wrong keywords. Using everyday language ("the molecules vibrate faster") instead of mark-scheme language ("particles gain kinetic energy").
- Reasoning before claim. Burying the claim at the end of the answer; markers may miss it.
How to drill CER
Take 10 past-paper OEQs. For each, rewrite the model answer in clear CER format with each part labelled. Then cover the model and rewrite from scratch. Then mark against the official scheme. Repeat with 10 fresh questions weekly.
For a tighter explainer of why students lose marks in OEQ, see why students lose marks in OEQ. For PSLE-specific OEQ technique, see how to improve PSLE Science OEQ. For subject-specific OEQ strategies, see Biology OEQ keywords + structure and Physics OEQ formulas + reasoning.