CER Writing
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The CER framework (Claim-Evidence-Reasoning) is a structured approach to scientific argumentation. Scientists use this framework to communicate findings, and it is essential for lab reports, research papers, and standardized test responses.
The Three Components of CER
Claim
A claim is a statement that answers a scientific question or addresses a problem. It should be:
- A complete sentence
- Specific and focused
- Directly answering the question asked
- Based on data (not a guess or opinion)
Example Question: Which fertilizer produces the tallest plants?
Strong Claim: Fertilizer B produces the tallest plants compared to Fertilizer A and the control group.
Weak Claim: Plants grow better with fertilizer. (Too vague, doesn't answer which one)
Evidence
Evidence is scientific data that supports your claim. Effective evidence is:
- Specific and quantitative when possible
- Directly related to the claim
- From your experiment or reliable sources
- Sufficient (multiple data points strengthen your argument)
Strong Evidence: "Plants treated with Fertilizer B had an average height of 24.5 cm after 4 weeks, compared to 18.2 cm for Fertilizer A and 12.8 cm for the control group."
Weak Evidence: "The plants with Fertilizer B were taller." (Not specific, no data)
Reasoning
Reasoning explains WHY your evidence supports your claim by connecting it to scientific principles. Reasoning should:
- Link evidence to the claim using scientific concepts
- Explain the underlying science
- Show understanding of cause and effect
- Address why alternative explanations are less likely
Strong Reasoning: "Fertilizer B contains a higher concentration of nitrogen, which is essential for chlorophyll production and protein synthesis. Increased nitrogen availability promotes cell division and elongation in plant stems, resulting in greater overall height. The control group's shorter height demonstrates that the additional nutrients, not just water and sunlight, were responsible for the increased growth."
Common CER Mistakes to Avoid
- Restating the evidence as reasoning: Reasoning must explain WHY, not repeat WHAT
- Using opinions instead of data: "I think" or "I believe" weakens scientific arguments
- Missing the scientific principle: Connect to concepts like energy transfer, cellular processes, etc.
- Insufficient evidence: One data point is rarely enough to support a claim
- Claim doesn't match evidence: Make sure your data actually supports what you're claiming
CER in ACT Science
The ACT Science section frequently asks you to evaluate claims based on data, identify which evidence supports a conclusion, and determine valid reasoning. Practice CER thinking to excel on these questions.
Examples
Example 1: Complete CER Response
Question: Based on your experiment, does water temperature affect the rate at which sugar dissolves?
Claim: Water temperature significantly affects the rate at which sugar dissolves, with higher temperatures resulting in faster dissolution.
Evidence: In our experiment, sugar dissolved completely in 45 seconds in water at 80C, 78 seconds at 50C, and 156 seconds at 20C. Each trial was repeated three times with consistent results (standard deviation less than 5 seconds for each temperature).
Reasoning: Higher temperatures increase the kinetic energy of water molecules, causing them to move faster and collide more frequently with sugar molecules. These collisions break the bonds between sugar molecules more quickly, allowing them to disperse into the solution. The consistent pattern across all trials indicates that temperature, not random variation, is responsible for the observed differences in dissolution rate.
Example 2: Evaluating a CER Response
Student Response: "My claim is that exercise increases heart rate. My evidence is that my heart rate went up when I exercised. My reasoning is that exercise increases heart rate because your heart beats faster."
Analysis:
- Claim: Acceptable but could be more specific
- Evidence: Weak - no specific data (what was the heart rate before and after?)
- Reasoning: Circular - just restates the claim. Should explain WHY heart rate increases (muscles need more oxygen, heart pumps faster to deliver oxygenated blood, etc.)
Improved Response: "Exercise increases heart rate. My resting heart rate was 72 bpm, which increased to 145 bpm after 5 minutes of jogging. Heart rate increases during exercise because working muscles require more oxygen for cellular respiration. The heart must pump blood faster to deliver this oxygen and remove carbon dioxide produced as a waste product."
Example 3: Identifying the Best Evidence
Claim: The enzyme works best at pH 7.
Which evidence best supports this claim?
- The enzyme was most active at pH 7.
- Enzymes are proteins that catalyze reactions.
- The reaction produced 15 mL of product at pH 7, compared to 8 mL at pH 5 and 6 mL at pH 9.
- pH affects enzyme activity.
Answer: C - This provides specific, quantitative data that directly supports the claim. Options A and D are too vague, and option B is a general fact unrelated to the specific claim.
Practice
Practice constructing and evaluating CER arguments with these exercises.
1. Write a claim to answer this question: "Does the type of surface affect how far a ball rolls?"
2. A student claims: "Brand X batteries last longer than Brand Y." Write two pieces of evidence that would support this claim.
3. Identify what's wrong with this reasoning: "The plant grew taller with more sunlight. This proves that sunlight makes plants grow taller because the plant was taller."
4. Data shows that ice melts faster in saltwater than freshwater. Write the reasoning component explaining WHY this occurs (hint: consider freezing point depression).
5. Evaluate this evidence: "The medicine worked because my headache went away after I took it." What makes this evidence weak? What additional information would strengthen it?
6. Write a complete CER response for this scenario: Students tested whether the color of light affects plant growth. Plants under red light grew 12 cm, blue light grew 15 cm, green light grew 5 cm, and white light grew 14 cm over 3 weeks.
7. A scientist claims that a new drug reduces blood pressure. What evidence would you need to see to be convinced? List at least three requirements.
8. Convert this weak claim into a strong claim: "Temperature affects stuff."
9. Students measured reaction times before and after drinking caffeinated soda. Before: average 0.45 seconds. After: average 0.38 seconds. Write the claim and evidence portions of a CER response.
10. Why is it important to include reasoning in scientific arguments, not just claim and evidence?
11. A CER response states: "Plants need water to survive. All 10 plants without water died within 2 weeks. Water is absorbed by roots and transported to cells where it is used in photosynthesis and maintains cell turgor pressure." Label each sentence as Claim, Evidence, or Reasoning.
12. Write a CER response explaining why ice floats on water. Include at least one specific piece of evidence and connect your reasoning to molecular structure.
Check Your Understanding
Q1: What is the difference between evidence and reasoning in a CER response?
Show Answer
Evidence is the specific data or observations that support your claim. Reasoning explains WHY that evidence supports the claim by connecting it to scientific principles and concepts. Evidence answers "What did you observe?" while reasoning answers "Why does this matter?"
Q2: What makes a claim "strong" in scientific writing?
Show Answer
A strong claim is specific (not vague), directly answers the question being asked, is based on data rather than opinion, and can be supported by evidence. It should be a complete sentence that takes a clear position.
Q3: Why is quantitative evidence generally stronger than qualitative evidence?
Show Answer
Quantitative evidence provides specific, measurable data that can be compared, analyzed, and verified. It reduces ambiguity and allows others to evaluate the strength of your argument more objectively. "The temperature increased by 15C" is more convincing than "it got hotter."
Next Steps
- Practice writing CER responses for your lab reports
- Review ACT Science passages and identify claims, evidence, and reasoning
- Move on to the Unit Checkpoint to assess your learning