Lesson 1.5 · Unit 1 · NGSS practice 7
Claim, evidence and reasoning: the structure biology grades
Most biology answers that lose marks are not wrong. They are incomplete in the same place every time: the student states an answer, quotes a number, and stops, leaving the reader to work out why the number supports the answer. That missing sentence has a name, and writing it deliberately is a learnable skill.
- The claim answers the question in one sentence. It is a position, not a description of what you did.
- The evidence is specific data that bear on the claim: values, units, comparisons, and where they came from. "The results showed a difference" is not evidence.
- The reasoning states the biological principle that makes the evidence count as support. This is the sentence students omit.
- Evidence must be selected, not dumped. Quoting every number you collected is not evidence; choosing the ones that bear on the claim is.
- A strong answer names what the evidence cannot show. Stating a limit is not weakness; it is the difference between a conclusion and an overclaim.
- The test for reasoning: could a reader who accepts your numbers still reject your claim? If so, the reasoning is missing.
Where students lose marks: restating the evidence in different words and calling it reasoning. "The treated group was taller, which shows the fertilizer made them taller" repeats the data. Reasoning explains the mechanism that connects the two.
The task. Using the river data from lesson 1.4, answer: what happened to dissolved oxygen downstream of the outfall, and why?
Step one: write a weak answer first, so the gap is visible. "Oxygen went down and then went back up because of the sewage." This has a claim buried in it, almost no evidence, and no mechanism.
Step two: write the claim. "Dissolved oxygen fell sharply immediately downstream of the outfall and recovered gradually over the following fourteen kilometers."
Step three: select the evidence. Three figures carry the argument: 8.2 mg/L at the outfall, a minimum of 2.4 mg/L at 2 km, and 8.0 mg/L by 16 km. The intermediate values at 1, 4 and 8 km support the shape but are not all needed.
Step four: write the evidence sentence with units. "Oxygen fell from 8.2 mg/L at the outfall to a minimum of 2.4 mg/L at 2 km, a loss of 5.8 mg/L, then rose to 8.0 mg/L by 16 km."
Step five: write the reasoning, naming the biology. "Sewage supplies organic matter that bacteria use as an energy source. Aerobic respiration by the growing bacterial population consumes dissolved oxygen faster than it dissolves in from the atmosphere, so the concentration falls. Downstream the organic matter has been used up, bacterial numbers and oxygen demand fall, and reaeration restores the concentration."
Step six: apply the test. Could a reader accept the three figures and still deny the claim? Before step five, yes: the numbers alone are equally consistent with a change in water temperature. After step five the mechanism links the outfall to the shape, and the argument holds together.
Step seven: state the limit. "These data are consistent with the outfall causing the sag but do not establish it, since no upstream or comparison measurements were taken and no variable was controlled." This sentence earns credit rather than losing it.
Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.
- Name the three parts of the structure.
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Claim, evidence, reasoning
- Which part do students most often leave out?
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The reasoning
- What makes a statement evidence rather than description?
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Specific values with units, selected because they bear on the claim
- State the test for whether reasoning is present.
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Could a reader accept the evidence and still reject the claim? If so, reasoning is missing
- Why does naming a limit earn credit?
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It distinguishes a supported conclusion from an overclaim
- Explain why "the treated plants were taller, which shows the fertilizer worked"
contains no reasoning.
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The second clause repeats the first in different words rather than connecting it to any biology. It does not say what fertilizer contains, what a plant uses that substance for, or why a shortage of it would limit height, so a reader who accepts the heights is given no principle for preferring the fertilizer explanation over better light or better drainage. Reasoning would supply that principle: nitrogen is needed to build amino acids and proteins, and where soil nitrogen limits growth, adding it allows more tissue to be built. It restates the evidence instead of naming the mechanism
- Write a claim, evidence and reasoning answer for the germination data in lesson
1.2 (88 percent at 20 degrees, 94 percent at 30 degrees).
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Claim: germination was higher at the warmer temperature in this trial. Evidence: 47 of 50 seeds germinated at 30 degrees Celsius, which is 94 percent, against 44 of 50 at 20 degrees, which is 88 percent, a difference of 6 percentage points. Reasoning: germination depends on enzyme-controlled reactions that mobilize the seed's stored food, and raising the temperature toward the enzymes' optimum increases the frequency of successful collisions between enzyme and substrate, so the reactions proceed faster and more seeds reach germination within the five day window. Limit: with one dish per temperature there is no measure of variation between dishes. Claim, then figures with units, then the enzyme mechanism, then the limit
- Why is quoting every number collected a weakness rather than thoroughness?
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Because selecting evidence is itself part of the argument, and a reader cannot tell which figures you think matter if you supply all of them equally. An answer that lists twelve values has not shown that it knows which two carry the claim, and it buries the comparison that does the work. Choosing the starting value, the minimum and the recovery value demonstrates understanding of the shape of the result, whereas reproducing the table demonstrates only that the table exists. Selection shows which data bear on the claim; a data dump hides it
- A student claims a pond is polluted because they found few fish. Identify what
is missing and supply it.
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The claim has almost no evidence and no reasoning. Evidence would need specific figures, such as the number of fish per sampling effort compared with a similar unpolluted pond, along with a measurement bearing directly on pollution such as dissolved oxygen or nitrate concentration. Reasoning would then have to connect them: many pollutants raise the growth of bacteria or algae whose respiration lowers dissolved oxygen below the level fish require, so a low oxygen reading alongside low fish numbers supports the claim in a way that fish numbers alone cannot. Both a comparative measurement and the oxygen mechanism linking it to fish numbers
- Explain why a good answer says what its evidence cannot establish.
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Because the strength of a conclusion is part of the conclusion, and claiming more than the data support is a specific error rather than enthusiasm. Observational data can show that two things occur together but cannot by themselves rule out a third factor or a reversed direction of cause, and an answer that says so is demonstrating exactly the judgment the question tests. It also directs the reader to what would settle the matter, which is the next step in any real investigation. Stating the strength of a conclusion is part of drawing it