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How are valid and reliable biological experiments designed?

Experimentation: observational versus experimental studies, controls, placebos and blinding, randomisation, replication and sampling, in vivo, in vitro and in situ studies, and the treatment of error and uncertainty.

An SQA Advanced Higher Biology answer on experimentation, covering observational versus experimental studies, positive and negative controls, placebos and blinding, randomisation, replication and sampling, in vivo, in vitro and in situ approaches, and the treatment of random and systematic error and uncertainty.

Generated by Claude Opus 4.812 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this key area is asking
  2. Observational and experimental studies
  3. Controls, placebos and blinding
  4. Randomisation, replication and sampling
  5. In vivo, in vitro and in situ
  6. Error and uncertainty
  7. Examples in context
  8. Try this

What this key area is asking

The SQA wants you to explain how valid and reliable experiments are designed: the difference between observational and experimental studies, the use of controls, placebos and blinding, randomisation, replication and sampling, the choice between in vivo, in vitro and in situ approaches, and how error and uncertainty are handled.

Observational and experimental studies

Controls, placebos and blinding

Randomisation, replication and sampling

In vivo, in vitro and in situ

Error and uncertainty

Examples in context

Example 1. The placebo in a drug trial. Giving the control group an identical dummy pill, with double-blinding, ensures that any improvement in the treated group is due to the drug and not to expectation or researcher bias. The example shows several design features combining to protect validity.

Example 2. In vitro versus in vivo testing. A new compound is first tested in vitro on cultured cells for tight control, then in vivo in animals for realism before human trials. The example shows the trade-off between control and realism that determines which approach is used at each stage.

Try this

Q1. State the purpose of a negative control. [1 mark]

  • Cue. To provide a baseline showing what happens without the treatment.

Q2. Explain why replication improves the reliability of results. [2 marks]

  • Cue. Repeating lets variability be estimated and random error be averaged out, so the mean is more dependable.

Exam-style practice questions

Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

SQA AH style4 marksExplain the purpose of a negative control, a placebo and double-blinding in an experiment.
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A 4-mark answer needs the purpose of each feature.

A negative control receives no treatment, or a treatment known to have no effect, and provides a baseline showing what happens without the independent variable, so any effect in the treated group can be attributed to the treatment.

A placebo is a dummy treatment given to a control group in medical and behavioural studies, so that the effect of the actual treatment can be separated from the effect of simply being treated.

Double-blinding means neither the participants nor the researchers know who received the treatment, which removes bias from both participant expectation and researcher assessment.

Markers reward (1) a negative control gives a baseline, (2) a placebo separates the treatment from the act of being treated, (3) double-blinding removes participant and researcher bias.

SQA AH style3 marksDistinguish between random and systematic error, and explain how each can be reduced.
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A 3-mark answer needs both errors and how to reduce them.

Random error causes scatter in repeated measurements with no consistent direction; it can be reduced by repeating measurements and taking a mean, which is improved by replication.

Systematic error shifts all measurements in the same direction, for example from an uncalibrated instrument; it can be reduced by calibrating instruments and standardising the technique.

Markers reward (1) random error is unpredictable scatter reduced by repetition and averaging, and (2) and (3) systematic error is a consistent shift reduced by calibration or standardising the method.

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