Experimental Design, Variables and Risk
Turning a biological question into a valid investigation with a testable hypothesis, controlled variables, suitable controls, replication and proportionate risk management.
How to study A-level Biology
Move between scales deliberately: identify the biological process, trace matter or information, use quantitative evidence, then evaluate the method and conclusion.
Core concepts
Concept 1
A valid investigation changes or compares the independent variable, measures the dependent variable and controls relevant confounding variables.
Exam cue: State the biological reason each controlled variable could affect the dependent variable.
Concept 2
Controls, repeats, replication, randomisation and blinding address different threats to a trustworthy inference.
Exam cue: Distinguish repeated measurements within a sample from independent biological replicates.
Concept 3
A method should be ethical, reproducible and supported by a risk assessment that identifies hazards, risks and control measures.
Exam cue: Link every method improvement to the specific validity, reliability, precision or safety problem it addresses.
Risk pitfalls and guardrails
Calling a variable controlled without saying how or why it matters.
Guardrail: Do not replace a mechanism with a vague purpose statement, or infer causation from an association without experimental support.
Treating more readings as a cure for systematic error.
Guardrail: Do not replace a mechanism with a vague purpose statement, or infer causation from an association without experimental support.
Confusing a hazard with the likelihood and severity of harm.
Guardrail: Do not replace a mechanism with a vague purpose statement, or infer causation from an association without experimental support.
Memory anchors
Independent Variable
The factor deliberately changed or used to define comparison groups.
Dependent Variable
The response measured to test the hypothesis.
Control Variable
A relevant factor kept constant so it does not confound the comparison.
Biological Replicate
An independent organism, culture or sample that captures biological variation.
Risk Assessment
Identify hazards, estimate risk and apply proportionate control measures before practical work.
Checkpoint rule
Do the check-up only after you can summarize each concept in one sentence and identify one dangerous pitfall from memory.
Knowledge Check (after reading)
Short check-up to confirm understanding of this module.
Check-up Questions
A student tests whether nitrate concentration affects duckweed growth. Which variable should be the dependent variable?
Six genetically different plants receive each treatment. Why are the six plants biological replicates?
Answer all questions to submit.
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Move forward only after this module is stable.
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