Confounding Variables

Travis DixonResearch Methodology

To avoid confounding variables you want to make sure all conditions of your experiment are equal.

Sometimes factors other than the IV may influence the DV in an experiment. These unwanted influences are called confounding variables. In laboratory experiments, researchers attempt to minimize their influence by carefully designing their experiment so all conditions are exactly the same – the only thing that’s different is the independent variable.

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Here are some confounding variables that you need to be looking out for in experiments:

  • Order Effects
  • Participant variability
  • Social desirability effect
  • Hawthorne effect
  • Demand characteristics
  • Evaluation apprehension

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ORDER EFFECTS: In repeated measures experiments one must be careful of order effects. Sometimes the order in which a participant does a task may alter the results. For instance, they may get better with practice and this could disrupt the results, or they remember something from the first condition that may alter their results.

Counterbalancing is one way of controlling for order effects. Counterbalancing is when repeated measures is used but half the group do Condition A then Condition B and the other half do it in the opposite order. Using an independent samples design also controls for order effects.

IB Psych IA Tips: When explaining your Design in the IB Psych IA, try to identify one or more extraneous variables you’re controlling for. 

PARTICIPANT VARIABILITY is the extent to which participants are different and is another potential factor that could influence an experiment’s results. For instance, in a study on the effects of a new training technique on fitness levels the existing fitness of the participants might be quite varied. This can easily be controlled for though either using random allocation or a matched pairs design.

DEMAND CHARACTERISTICS are the cues in a study (characteristics) that may lead the participant to figure out how they’re supposed to act (according to the demands of the researcher/experiment). It leads to participants behaving in a way that they think they’re supposed to, not how they would naturally.

The placebo effect is a type of participant expectancy effect.

PARTICIPANT EXPECTANCY EFFECT is the name given to the change in behaviour as a result of participants behaving in a way that they think they’re expected to. In other words, demand characteristics in an experiment’s design might lead to participant expectancy effect occurring. These terms are commonly used in correctly (Read more: Demand characteristics: What are they really?)

SOCIAL DESIRABILITY EFFECT is which is when people change their behaviour because they have a nature desire to be liked by other people. Another factor that influences people’s behaviour is when they don’t act like they normally would simply because they are being watched by someone. This was first recorded in a study on the Hawthorne Electrical Plant in the USA and has become known as the HAWTHORNE EFFECT. In the original Hawthorne Plan research they found the workers were working harder simply because they were being watched. Doesn’t this happen in the classroom? Suddenly when the teacher starts walking around the room checking work you close youtube, put away your phone, tuck away the love-letter, etc etc.

The terms “confounding variable” and “extraneous variable” are used interchangeably. Technically speaking, an extraneous variable is any variable that could affect the results, whereas “Confounding occurs when the influence of extraneous variables on the DVs cannot be separated and measured,” (Street et al. 1995)

EVALUATION APPREHENSION might occur when participants are anxious about being evaluated on a particular task or skill (sometimes called the spotlight effect). This might change their behaviour. Think about your oral assignments in some of your subjects, for instance. If you weren’t being graded you might be OK talking in front of your class but as soon as your teacher gets out their big red pen and beings giving you a grade on your work you’re likely to become nervous and this will affect your performance. People are often nervous about being in an “experiment” because the word might conjure many scary thoughts.

Psychologists must balance validity, practicality and ethicality when designing experiments.

Some textbooks also mention maturation – when participants get better on the second or third trial simply because they have practiced the skill (like order effects). Information contamination is another term sometimes used. This is when outside information affects the results of the experiment.


You don’t have to know a confounding variable by name to evaluate an experiment. The following “experiment” has the independent variable of chewing gum. However, there are many flaws in this experiment. These flaws raise issues about the experiment’s validity (it’s really a commercial for gum so it’s heavily biased). What confounding variables and/or methodological limitations can you find in this experiment?


Street, D. L. (1995). Controlling extraneous variables in experimental research: a research note. Accounting Education, 4(2), 169–188.