CER Writing
Learn
This lesson teaches the Claim-Evidence-Reasoning (CER) framework for scientific argumentation. While the ACT Science section is multiple choice, understanding CER helps you evaluate which answer choices have proper scientific support and reasoning.
The CER Framework
- Claim: A statement that answers the question or makes an assertion
- Evidence: Specific data, observations, or facts that support the claim
- Reasoning: Explanation of how and why the evidence supports the claim, connecting to scientific principles
Why CER Matters for ACT Science
ACT Science questions often ask you to:
- Determine which conclusion is supported by the data
- Identify which evidence supports or contradicts a hypothesis
- Evaluate the reasoning behind scientific arguments
- Choose the best explanation for experimental results
Components in Detail
Strong Claims
- Directly address the question
- Are specific and testable
- Do not overreach beyond what the data shows
Strong Evidence
- Comes directly from data, graphs, or tables
- Is specific (includes numbers, measurements, observations)
- Is accurate and correctly interpreted
Strong Reasoning
- Explains the connection between evidence and claim
- Uses scientific principles or concepts
- Shows logical thinking
- Addresses why the evidence matters
Common CER Mistakes
- Weak Claim: Too vague or not answering the actual question
- Insufficient Evidence: No data cited, or wrong data selected
- Missing Reasoning: Evidence stated but no explanation of why it matters
- Overclaiming: Concluding more than the data actually supports
Examples
Example 1: Complete CER Response
Question: Based on the data, does fertilizer concentration affect plant growth?
Data: Plants with 0g fertilizer grew 5cm; 5g fertilizer grew 12cm; 10g fertilizer grew 18cm; 15g fertilizer grew 15cm.
Claim: Fertilizer concentration does affect plant growth, with optimal growth occurring at 10g concentration.
Evidence: Plants grown with 10g of fertilizer reached 18cm, compared to only 5cm with no fertilizer. However, increasing to 15g resulted in reduced growth (15cm).
Reasoning: The data shows a positive relationship between fertilizer and growth up to 10g, suggesting nutrients enhance growth. The decrease at 15g indicates that excess fertilizer may become harmful, possibly due to nutrient toxicity or salt buildup affecting root function.
Example 2: Evaluating Answer Choices Using CER
Question: Which conclusion is best supported by the experimental data?
Data shows enzyme activity increases from 20C to 40C, then decreases from 40C to 60C.
A. Enzymes work best at high temperatures.
B. Enzyme activity is highest at 40C and decreases at higher temperatures.
C. Temperature has no effect on enzyme activity.
D. Enzymes are destroyed above 40C.
Analysis: Choice B is correct because it accurately reflects the data (peak at 40C, decrease after). Choice A overclaims - activity decreased at 60C. Choice C contradicts the data. Choice D overclaims - we know activity decreased but cannot conclude destruction without additional evidence.
Example 3: Identifying Weak Reasoning
Claim: Brand X battery lasts longer than Brand Y.
Evidence: In a test, Brand X lasted 8 hours and Brand Y lasted 6 hours.
Weak Reasoning: "Brand X is better because it lasted longer."
Why It's Weak: This reasoning just restates the evidence. Strong reasoning would explain WHY: "The 2-hour difference (33% longer life) indicates Brand X has either higher energy density or more efficient power delivery, making it a better choice for extended use."
Practice
Apply the CER framework to these scenarios.
1. Data shows that plants in red light grew 15cm, blue light grew 22cm, green light grew 8cm, and white light grew 20cm. Write a claim about which light color is best for plant growth. Cite specific evidence and explain your reasoning.
2. An experiment tests reaction time after drinking caffeine vs. water. Caffeine group averaged 0.25 seconds; water group averaged 0.35 seconds. A student concludes "Caffeine makes you faster at everything." Evaluate this claim - is it overclaiming? What would be a better claim?
3. A scientist claims that Species A is more heat tolerant than Species B. What evidence would you need to see to support this claim? List at least three specific data points or observations.
4. Examine this response: "The medicine works because patients felt better." Identify what components of CER are missing or weak. Rewrite it as a stronger CER response.
5. Data shows dissolved oxygen levels: Lake A (8 mg/L), Lake B (4 mg/L), Lake C (12 mg/L). Lake C has the most fish species. Write a CER paragraph explaining the relationship between oxygen and biodiversity.
6. A graph shows that as elevation increases, average temperature decreases. At 0m, temperature is 25C. At 1000m, temperature is 19C. At 2000m, temperature is 13C. Write a claim, cite evidence, and provide reasoning that includes the rate of temperature change.
7. Two students interpret the same data differently. Student A says "pH affects bacterial growth." Student B says "Bacteria grow best at neutral pH." The data shows: pH 4 (10 colonies), pH 7 (95 colonies), pH 10 (15 colonies). Which claim is better supported? Explain using CER principles.
8. An ACT question asks which conclusion is supported by data showing plant height over time. The data shows rapid growth weeks 1-3, slower growth weeks 4-5, and no growth weeks 6-8. Evaluate these answer choices: A) Plants stop growing after week 5. B) Plant growth rate varies over time. C) Plants need more water after week 3. D) Plants reached maximum height by week 6.
9. A scientist observes that ice cubes melt faster in salt water than fresh water. Write a CER response that includes reasoning about why this occurs at the molecular level.
10. Data from a genetics experiment shows: Cross 1 produces 75% tall plants, 25% short plants. Cross 2 produces 100% tall plants. Cross 3 produces 50% tall, 50% short. Make a claim about the inheritance pattern and support it with evidence and reasoning about dominant and recessive alleles.
Check Your Understanding
Review your understanding of CER with these questions.
1. What is the difference between evidence and reasoning in a CER response?
Show Answer
Evidence is the specific data or observations from the experiment. Reasoning explains WHY and HOW that evidence supports the claim, connecting it to scientific principles or logic.
2. What does it mean to "overclaim" and why is it a problem?
Show Answer
Overclaiming means making a conclusion that goes beyond what the data actually supports. It's a problem because science requires conclusions to be directly supported by evidence. On the ACT, overclaiming answer choices are designed to trap students.
3. How can understanding CER help you on multiple choice ACT Science questions?
Show Answer
It helps you evaluate answer choices by checking if they make appropriate claims based on the evidence provided. You can eliminate answers that overclaim, contradict data, or lack logical reasoning.
Next Steps
- Practice writing CER responses for data you encounter in science class
- When reviewing ACT practice tests, analyze why wrong answers fail the CER test
- Move on to the final lesson: Unit Checkpoint