Control Methods

Control in Psychological Research

A well-designed experiment isolates the effect of the independent variable (IV) by ensuring that all other variables — extraneous variables — are either held constant or distributed equally across conditions. If extraneous variables are not controlled, they may systematically affect the dependent variable (DV) and offer alternative explanations for any observed effect, reducing the study's internal validity. Four key control techniques are used to achieve this: random allocation, counterbalancing, randomisation, and standardisation.

Random Allocation

Random allocation (random assignment) is the process of assigning participants to experimental conditions by chance — for example, using a random number generator, drawing names from a hat, or tossing a coin. Every participant has an equal probability of being assigned to any condition. Random allocation is used in independent groups designs.

Random allocation controls for participant variables — pre-existing differences between people (age, intelligence, personality, prior experience) that might affect the DV. By randomly distributing participants across conditions, random allocation makes it statistically unlikely that the groups will differ systematically on any such variable. It does not eliminate individual differences; it distributes them equally across conditions so they cannot explain any between-group difference in the DV.

Counterbalancing

Counterbalancing is used in repeated measures designs to control for order effects — the influence of completing one condition before another on performance in the second condition. Order effects include: practice effects (improved performance on the second condition due to familiarity with the task) and fatigue effects (poorer performance on the second condition due to tiredness or boredom). In an ABBA counterbalancing scheme, half the participants do Condition A then B; the other half do B then A. This ensures that any order effect operates equally across both conditions.

Randomisation

Randomisation refers to the random ordering of stimuli, trials, or items within a condition — for example, presenting words in a memory experiment in a different random order for each participant, or randomising the sequence in which stimuli appear. This prevents systematic sequencing effects (where particular items always appear early or late, consistently affecting performance) from biasing results. Randomisation distributes the effects of stimulus order across all participants rather than allowing them to systematically favour or disadvantage any condition.

Standardisation

Standardisation involves using identical procedures, instructions, materials, and conditions for all participants. A standardised procedure ensures that all participants experience exactly the same research environment — the same wording of instructions, the same timing, the same apparatus, and the same physical conditions. This prevents investigator variability (differences in how the researcher behaves with different participants) from becoming a confound. Standardised instructions also reduce demand characteristics by ensuring the researcher gives no inadvertent cues about expected responses.

 Key Takeaways

  • Extraneous variables threaten internal validity — control techniques prevent them becoming confounds that offer alternative explanations for DV changes.
  • Random allocation: participants assigned to conditions by chance — distributes participant variables equally across groups in independent designs.
  • Counterbalancing: controls order effects in repeated measures — half do A then B, half do B then A (ABBA), so practice and fatigue operate equally across conditions.
  • Randomisation: random ordering of stimuli/trials — prevents systematic sequencing effects from biasing any condition.
  • Standardisation: identical procedures, instructions, and materials for all participants — prevents investigator variability and reduces demand characteristics.
  • Key distinction: random allocation (who goes in which group) vs randomisation (order of stimuli within a study). Both use chance but control different sources of bias.