Grade: Grade 12 Subject: Science Unit: Inquiry & Research Lesson: 4 of 6 SAT: ProblemSolving+DataAnalysis ACT: Science

CER Writing

Learn

The Claim-Evidence-Reasoning (CER) framework is the standard structure for scientific writing. This lesson teaches you how to construct clear, evidence-based arguments that communicate your research findings effectively.

The CER Framework

Component Definition Key Question
Claim A statement that answers the research question What do you conclude?
Evidence Data or observations that support the claim What data supports this?
Reasoning Scientific principles that explain why the evidence supports the claim Why does this evidence matter?

Writing Strong Claims

  • Directly answer the research question
  • Be specific and measurable when possible
  • Avoid vague language like "it worked" or "there was a change"
  • Include the relationship between variables

Presenting Evidence Effectively

  • Use specific data points, not generalizations
  • Include units of measurement
  • Reference your data tables or graphs
  • Use multiple pieces of evidence when available
  • Acknowledge data limitations or anomalies

Constructing Reasoning

  • Connect evidence to scientific principles or theories
  • Explain the cause-and-effect relationship
  • Show why the evidence supports your claim, not just that it does
  • Use appropriate scientific vocabulary
  • Address potential alternative explanations

Common CER Mistakes

  • Restating evidence as reasoning: "The data shows this because the data shows this"
  • Using opinion instead of evidence: "I think this happened because..."
  • Vague claims: "The experiment worked" instead of "Plant height increased by 45%"
  • Missing the connection: Evidence and reasoning that do not logically link to the claim

Examples

Example 1: Complete CER Paragraph

Research Question: How does fertilizer concentration affect tomato plant growth?

Claim: Moderate fertilizer concentration (50%) produces the greatest tomato plant growth compared to no fertilizer or full-strength fertilizer.

Evidence: After 4 weeks, plants receiving 50% fertilizer had a mean height of 28.4 cm, compared to 18.2 cm for the control group (0%) and 22.1 cm for the 100% group. The 50% group also produced 40% more leaves on average (see Figure 2).

Reasoning: Plants require nitrogen, phosphorus, and potassium for cellular growth and photosynthesis. At 50% concentration, plants received adequate nutrients to maximize growth. The control group lacked sufficient nutrients, limiting cell division and leaf development. The 100% group likely experienced nutrient toxicity or osmotic stress, which inhibits water uptake and can damage root cells, explaining the reduced growth despite more available nutrients.

Example 2: Weak vs. Strong Evidence

Weak: "The plants with fertilizer grew more than the ones without."

Strong: "Plants in the 50% fertilizer group showed a mean height increase of 10.2 cm over 4 weeks, compared to 5.8 cm for the control group (Table 1). This represents a 76% greater growth rate."

Example 3: Weak vs. Strong Reasoning

Weak: "This shows the fertilizer helped the plants grow better."

Strong: "The increased growth in the fertilizer group is consistent with the role of nitrogen in protein synthesis and chlorophyll production. With more chlorophyll, plants can capture more light energy for photosynthesis, producing additional glucose for cellular respiration and growth."

Practice

Develop your CER writing skills with these exercises.

1. Identify the claim, evidence, and reasoning in this paragraph: "Water boils at a lower temperature at high altitudes. When water was heated at 3,000 meters elevation, it boiled at 90 degrees C instead of 100 degrees C. This occurs because atmospheric pressure decreases with altitude, and lower pressure reduces the energy needed for water molecules to escape into the gas phase."

2. Rewrite this weak claim to make it stronger: "The experiment showed that temperature affects enzyme activity."

3. This evidence statement lacks specificity: "The reaction happened faster when heated." Add specific data to make it scientific evidence.

4. Write the reasoning component for this claim and evidence: Claim: Salt lowers the freezing point of water. Evidence: Pure water froze at 0 degrees C, while a 10% salt solution froze at -6 degrees C.

5. Identify the error in this CER: "Claim: Exercise improves memory. Evidence: I felt like I remembered more after running. Reasoning: This proves exercise helps the brain."

6. Given this data set, write a complete CER paragraph: An experiment tested antibacterial soap vs. regular soap. Zone of inhibition: antibacterial soap = 2.3 cm mean; regular soap = 2.1 cm mean; water control = 0.0 cm.

7. A student writes: "The data supports my hypothesis because it matches what I predicted." Explain why this is poor reasoning and rewrite it properly for an experiment about light affecting plant growth.

8. Write a claim that would be appropriate for an experiment comparing the insulation properties of wool, cotton, and polyester fabrics.

9. Convert these raw observations into evidence for a CER: "Monday: 5 tadpoles had back legs. Tuesday: 8 had back legs, 2 had front legs. Wednesday: 10 had back legs, 5 had front legs. Thursday: all 12 had all legs."

10. Write a complete CER for this research question: "Does the color of light affect photosynthesis rate?" Use hypothetical but realistic data, and include reference to the light absorption properties of chlorophyll in your reasoning.

Check Your Understanding

Q1: What distinguishes reasoning from evidence in a CER paragraph?

Q2: Why is it important to include specific numerical data in your evidence?

Q3: How does reasoning connect evidence to scientific principles?

Q4: What makes a claim "testable" versus "opinion"?

Next Steps

  • Practice identifying CER components in published research articles
  • Revise previous lab reports using the CER framework
  • Continue to the Unit Checkpoint to test your understanding of all inquiry and research concepts