What is an experiment?

• What is an experiment?

  • Researcher attempts to measure the effect of changes of one thing on another i.e. try to establish cause and effect
  • This is done by manipulating the independent variable and and measuring the effect this has on the dependent variable
  • Also try to control extraneous variables to stop them affecting results of the experiment
• What is the experimental design?


  Describes how participants are put into different groups in an experiment.
• What are independent groups?

  • Where different participants are used in each condition of the independent variable
  • Simplest way to assign participants to a group
  • Don't have to worry about order effects (practice/demand characteristics)
  • People vary in a large amount of ways. Randomly assigning participants to different groups means important participant variables may be over-represented in one group and under-represented in the other.
  • Larger amount of participants needed
• What are repeated measures?

  • The same group of participants take part in each independent variable condition
  • No participant variables effect results
  • A smaller amount of participants are needed
  • By the time they take part in the second condition, participants may have guessed the aim of the study and display demand characteristics
  • By the time they take part in the second condition, participants may have become good at the task being measured or got bored with is. These order effects may affect their performance
• What are matched pairs?

  Experimenter finds a variable that could effect the results (e.g. age of participants) and pairs a participant up with another person who has the same variable as them. Then one member of each pair is put into each condition. By using matched pairs we can be sure that other possibly important differences between participants are equally represented within each group.
• Evaluation of matched pairs:

  • Because each participant takes part in only one condition, there can be no order effects (unlike repeated measures design)
  • Can be costly and time-consuming to match participants and then assign them to groups
  • Participant variables other than the one for which participants were matched could still affect the results
• What is counterbalancing?


  A technique used in the repeated measures design to reduce the effect of order effects. Half the participants do condition A, then B. The other half do B, then A. This means that overall the order effects cancel each other out.
• What is random allocation?

  The process of assigning participants to different groups or conditions in a study randomly, to ensure that each participant has an equal chance of being assigned to any group. Used to reduce effect of participant variables in an independent groups design.