Control: random allocation and counterbalancing, randomisation,…

The idea

Researchers use specific techniques to minimise bias and extraneous variables. Random allocation ensures every participant has an equal chance of being assigned to any condition, reducing participant variables. In repeated measures designs, counterbalancing controls for order effects by having half the participants complete condition A then B, and the rest B then A. Randomisation uses chance methods to determine the order of trials, reducing investigator effects. Standardisation ensures all participants experience exactly the same environment and instructions. Finally, control groups provide a baseline measure by receiving no experimental treatment, allowing researchers to isolate the true effect of the independent variable.

What earns the marks

• Describe random allocation as a method to evenly distribute participant differences across experimental conditions.
• Explain counterbalancing as a technique to mitigate order effects in repeated measures designs, typically using the ABBA method.
• Define standardisation as keeping all situational variables and procedures identical for every participant.
• Explain the purpose of a control group as establishing a baseline to compare against the experimental group.
• Identify randomisation as the use of chance to dictate the order of stimulus presentation or procedural steps.

Common mistakes and how to put them right

• Confusing random sampling with random allocation.

  Put it right: Random sampling selects the participants from a population, whereas random allocation assigns already selected participants to conditions.

• Stating that counterbalancing eliminates order effects.

  Put it right: It does not remove them, but rather balances their impact across the different conditions.

• Failing to specify how standardisation is achieved.

  Put it right: Simply stating 'keep things the same' is insufficient compared to specifying 'using standardised written instructions'.

Exam tips

• When designing an experiment in an exam, explicitly state how you would randomly allocate participants, such as drawing names from a hat.
• Use the term 'baseline' when explaining the purpose of a control group in evaluating the independent variable's effect.
• Specify exactly what is being standardised in a given scenario, such as the time of day or the exact wording of the brief.