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Biology · Grades 9–12

Biological investigations and causal claims

Use controls, replication and ethical boundaries.

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Design

Specify the question, changed variable, measured outcome and controlled conditions.

Replication

Independent replicates help characterize variation; repeated readings of one sample are not identical to independent samples.

Inference

Association alone does not establish cause; confounding and selection effects can matter.

Ethics

Use supplied data and non-harmful observations; studies involving people or animals require appropriate oversight.

Worked examples

Which is the measured dependent variable in a stated light-versus-plant-height study?

Height is the measured outcome in this design.

Plant height after the stated interval.

Do ten readings of one plant equal ten independently grown plants?

Technical repeats and biological replicates answer different variability questions.

No.

Check your understanding

  1. Independent measured heights are 8, 10 and 12 cm. Find the mean.

  2. Why should water supply be controlled when comparing light treatments?

  3. Does correlation between two variables alone establish causation?

    1. Only if the graph is colorful.
    2. Yes, any correlation proves cause.
    3. No.

Answers and marking notes

Answers and marking notes
  1. 10 cm

    (8 + 10 + 12)/3 = 10 cm.

  2. Water could affect growth and confound the light comparison.

    A valid control reduces alternative explanations.

  3. No.

    Confounding or reverse causation may explain an association.

Further reading and review references

These are reference links, not copied exercises, institutional endorsement or proof of independent review.

Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.