Writing
Claim, evidence, reasoning (CER): how to write one
Claim, evidence, reasoning (CER) is a three-part way to answer a question in writing. The claim is your answer, the evidence is the data, observations or quotations that support it, and the reasoning explains why that evidence proves the claim, often by applying a scientific rule or idea.
CER is used most in middle and high school science, where students write one up after a lab, but it works just as well in social studies, math and English. It grows out of older models of how arguments work, and it boils them down to three questions any student can answer: What do you think? How do you know? Why does that prove it?
What is claim, evidence, reasoning?
| Part | Question it answers | What it looks like |
|---|---|---|
| Claim | What is your answer? | One clear sentence that directly answers the question. |
| Evidence | How do you know? | Specific data, measurements, observations or quotations, with numbers and units where possible. |
| Reasoning | Why does the evidence prove the claim? | An explanation that links the evidence to the claim, usually through a rule, concept or principle you have studied. |
The three parts work like a bridge. The claim is where you want to go, the evidence is the solid ground you stand on, and the reasoning is the span that connects them. Many students write a strong claim and good evidence but skip the span, which is why reasoning is usually the part that loses points.
How to write a claim
- Answer the question that was asked. If the question is "Does temperature affect how fast sugar dissolves?", your claim must say yes or no, and how.
- Keep it to one sentence.
- Leave out "I think" and "I believe." Just state the answer.
- Make it specific. "Temperature matters" is weaker than "Sugar dissolves faster in hotter water."
- Keep the evidence out of the claim. The numbers come next.
How to choose evidence
Good evidence is relevant (it relates directly to the claim), specific (it gives numbers, units, names or exact words) and sufficient (there is enough of it to be convincing). In science, evidence usually comes from your own data table or graph. In English or history, it is a quotation or fact from a text, and you should cite where it came from (see MLA in-text citations). Two or three pieces of evidence are better than one. Outside sources should be trustworthy; see credible sources.
How to write reasoning
Reasoning answers the question "So what?" about each piece of evidence. It names the rule, concept or idea that explains why the evidence supports the claim. In a science class, that might be a principle from the unit, such as the fact that heat makes particles move faster. In an English class, it might be what a detail reveals about a character. If your reasoning only repeats the numbers, it is still evidence. Real reasoning explains why the numbers came out the way they did.
CER sentence frames
- Claim: "[Answer to the question] because [short reason]."
- Claim: "[Change you made] [increased / decreased / did not change] [result]."
- Evidence: "In our experiment, ..."
- Evidence: "The data show that ..., while ..."
- Evidence: "According to the graph, ..."
- Evidence: "On page [number], the author writes, '...'"
- Reasoning: "This evidence supports my claim because ..."
- Reasoning: "This happens because [scientific principle] ..."
- Reasoning: "Since [rule or concept], the results show that ..."
- Reasoning: "This detail matters because it shows that ..."
CER example: a science lab
Question: Does the temperature of water affect how fast sugar dissolves? A class tested this by stirring one teaspoon of sugar into 200 milliliters of water at three temperatures and timing how long the sugar took to disappear. Each time below is the average of three trials.
| Water temperature | Average time to dissolve (seconds) |
|---|---|
| Cold (10 °C) | 184 |
| Room temperature (22 °C) | 97 |
| Hot (60 °C) | 39 |
Claim: Sugar dissolves faster in hotter water.
Evidence: In our experiment, one teaspoon of sugar took an average of 184 seconds to dissolve in cold water at 10 °C, 97 seconds in room-temperature water at 22 °C, and only 39 seconds in hot water at 60 °C. Each rise in temperature cut the dissolving time, and the hot water was more than four times faster than the cold water.
Reasoning: Temperature is a measure of how fast particles are moving. In hot water, the water molecules move faster and strike the sugar crystals more often and with more energy, so they pull sugar molecules away from the crystals and spread them through the water more quickly. In cold water, the molecules move more slowly, so dissolving takes longer. This explains why the dissolving time dropped every time the water temperature went up.
CER example: reading and English class
CER also works for questions about a story. Here is a short passage written for this page, followed by a CER answer to the question "How does Rosa feel before the swim meet?"
Rosa checked her goggles for the fourth time. The strap was fine, but she pulled it tight again anyway. Around her, the other swimmers laughed and stretched. Rosa stared at the water and counted the lanes, then counted them again. When her coach called her name, she jumped as if someone had dropped a tray.
Claim: Rosa feels very nervous before the swim meet.
Evidence: Rosa checks her goggles "for the fourth time" even though "the strap was fine." She counts the lanes and then counts "them again," and when her coach calls her name, "she jumped as if someone had dropped a tray."
Reasoning: People who are anxious often repeat small actions to feel in control, and Rosa keeps checking things that do not need checking. Counting the lanes over and over shows that her mind is busy and restless. Her jump at the sound of her own name shows that she is tense and on edge. Together, these details point to nervousness rather than excitement, especially because she does not laugh or stretch like the swimmers around her.
Writing CER as one paragraph
Some teachers want the three parts labeled, as in the samples above. Others want one smooth paragraph. To turn CER into a paragraph, open with the claim as your topic sentence, present each piece of evidence, follow each one with reasoning, and finish with a concluding sentence. Some classes add a fourth part, a rebuttal, which names another possible explanation and shows why the evidence does not support it. That version is often called CERR, and it is a short step from there to a full argumentative essay.
Sugar dissolves faster in hotter water. In our experiment, a teaspoon of sugar took an average of 184 seconds to dissolve in water at 10 °C but only 39 seconds at 60 °C, with room-temperature water falling in between at 97 seconds. These results make sense because temperature measures how fast particles move. Faster water molecules strike the sugar crystals more often and with more energy, so they break sugar molecules loose more quickly. Since the time dropped at every step up in temperature, the data clearly show that heat speeds up dissolving.
CER checklist
- My claim answers the exact question in one sentence.
- I used at least two pieces of evidence.
- My evidence is specific: numbers with units, or exact quotations.
- My reasoning names a rule, concept or idea from class.
- My reasoning explains how each piece of evidence supports the claim.
- I did not add a new claim at the end.
Common CER mistakes
- Evidence that just repeats the claim. "Sugar dissolves faster in hot water because it dissolved faster" proves nothing.
- Reasoning that only restates the data. Explain why the numbers came out that way.
- A vague claim, such as "Temperature has an effect." Say what the effect is.
- Forgetting units. "It took 39" means nothing without "seconds."
- Using evidence that does not relate to the claim, such as the color of the sugar.
- Writing "I think" in the claim.
Check yourself
1. In CER, what does the reasoning do?
Answer: Explains why the evidence supports the claim. Reasoning connects the evidence to the claim, usually by applying a rule or concept.
2. Which is the strongest claim for the question "Do plants grow taller with more light?"
Answer: Plants grow taller when they get more light.. It answers the question directly in one sentence. The third option is evidence, not a claim.
3. Which sentence is evidence?
Answer: The plant by the window grew 12 centimeters, while the plant in the closet grew 2.. Evidence is specific data. The first sentence is reasoning, and the third is a claim.
4. What is a common sign that reasoning is too weak?
Answer: It only repeats the numbers from the evidence.. Reasoning must explain why the evidence proves the claim, not repeat the evidence.
Pick an answer to start.