Open the app
  1. Home
  2. Lessons
  3. GCSE Psychology
  4. Controlling extraneous variables

Controlling extraneous variables

🎬 The doodle video for this lesson is coming soon. Subscribe on YouTube to see it first.

Researchers use several techniques to control extraneous variables so that only the IV affects the DV.

Standardisation, counterbalancing and randomisation

Standardised procedures: every participant gets the same instructions, materials, setting and timing. Strength: reduces situational variables and lets others replicate the study. Weakness: can make the study artificial.
Counterbalancing: used in repeated measures designs. Half the participants do condition A then B; the other half do B then A. This balances out order effects (practice, boredom, tiredness). Strength: order effects affect both conditions equally. Weakness: it does not remove order effects, only spreads them.
Randomisation: using chance to decide things, such as which participants go in which group, or the order of items. Strength: reduces researcher bias and spreads participant variables evenly. Weakness: by chance, groups can still end up unequal.

Single-blind and double-blind

Single-blind: participants do not know which condition they are in, or the aim. Strength: reduces demand characteristics (participants guessing the aim and changing their behaviour). Weakness: the researcher still knows, so may influence results.
Double-blind: neither the participants nor the researcher who collects the data know which condition each participant is in. Strength: reduces both participant and researcher bias. Weakness: harder and more expensive to organise.
Example: in a drug trial, some people get the real tablet and some get a placebo (a fake). In a double-blind trial, neither they nor the person testing them knows which they got.

Choosing the right control

Match the problem to the control that fixes it.
Worked example

In a repeated measures study, all participants do a memory test in silence and then with music. Why is this a problem, and how could it be fixed?

  1. By the second test, participants may have improved with practice or become tired: order effects.
  2. Counterbalancing: half do silence first, half do music first.

Answer: Order effects could affect the results; counterbalancing balances them across conditions.

Key idea

Standardised procedures keep everything the same; counterbalancing balances order effects in repeated measures; randomisation uses chance to reduce bias; single-blind hides the condition from participants; double-blind hides it from participants and researchers.

The interactive lesson includes the diagrams for this topic.

Check you have got it

Answer 7 quick questions with instant marking. If you get one wrong, GCSE-ready shows you why and gives you another go. It is free, and you do not need an account.

Practise this lesson free (7 questions)All GCSE Psychology lessons