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Grade 6 · lesson 3 of 4

Spot Two Changes at Once

Explain why a design cannot isolate the tested factor.

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Supplied materials

  • Use the supplied fictional observations or model. Paper and pencil are optional; spoken answers are welcome. No equipment or real experiment is needed.

Learn the idea

A comparison is confounded when another changed condition could explain the outcome. Suppose the question is about light time, but the longer-light group also receives more water. A growth difference cannot be attributed to light alone from that design, even if the group means are far apart. Strong claims require attention to design as well as arithmetic.

Supplied fictional plan: group A receives 6 light-hours and 50 water-units daily; B receives 3 light-hours and 20 water-units. All seedlings begin at 4 cm. A growths are 4, 5, 6 cm and B growths 1, 2, 3 cm over one week. The means differ by 3 cm, but both light and water differ. Water-units are model labels; no growing is required.

  1. List every intentionally different condition.
  2. Compute the two growth means.
  3. Explain why the mean difference cannot isolate light.
  4. Repair the paper plan by giving both groups the same water amount while keeping light different.
Worked example

See the method in action

A averages 5 cm and B 2 cm: a 3 cm observed difference. Both light and water changed, so these records cannot identify light alone as the cause. A repaired plan uses equal water for both groups.

Your turn

Check your reasoning

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Which change best repairs the light comparison?

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See the answer and explanation

Give both groups 35 water-units and keep their different light times.

Equal water removes the supplied water confound while preserving the intended light contrast. Other relevant conditions must also be kept alike.

Take it further

Change the question to water amount instead. Set both groups to 4 light-hours but retain 50 versus 20 water-units; this revised plan compares water while holding light alike.

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These LifeStarter passages, scenarios and exercises are authored for this Academy with AI assistance and review. External resources keep their publishers’ terms. A grade label suggests a starting level; it does not establish alignment with your school’s complete requirements.