Experimental Design

• An experimenter needs to decide whether a participant will take part in one condition or both.
• This decision will determine the experimental design.
• There are three elements to experimental design.
• The three designs you need to be aware of are:

  • Independent Groups
  • Repeated Measures
  • Matched Pairs.
• Individual Groups Design-

  Ppts involved in the study are usually divided into two subgroups.
• One group will take part in the experimental conditions while the other group will form the control conditions.
• Each ppt will only complete one condition.
• Strengths of Independent Groups-

  Order Effects are not a problem with independent groups- this is when the order in which ppts take part in each condition have an impact on their behaviour.

  This is not a problem for independent groups as each ppt only takes part in one condition.
• Weaknesses of Independent Groups-

  Less economical- you need more ppts with independent groups. Twice as many as the repeated measures design.

  Increase the amount of time and money spent on the experiment.

  Individual differences between ppts could result in confounding variables.

  This could affect the validity of the experiment.
• Repeated Measures Design-

  All the ppts take part in all the conditions.
• The results would then be compared between the conditions to assess what impact the IV had on the DV.
• In experiments where there was numerous different conditions, the same ppts would be used across them while exposed to different independent variables.
• Repeated Measures- Evaluation

  Strengths:

  Ppts variables are controlled so there is a higher degree of validity than independent groups.

  Cheaper and more time efficient than independent groups.
• Weaknesses-

  Order Effects could result in demand characteristics or even boredom from completely two similar tasks.

  Completing the same task twice could also result in ppts gaining practice.
• Matched Pairs Design-

  Ppts are studied prior to the experiment based on certain characteristics- relevant to the study.
• The test allows ppts to be matched with someone similar to them.
• One ppt in each pair will take part in each condition.
• Results are then compared as if they were gathered from one individual.
• Matched Pairs- Evaluation

  Strengths:

  Order Effects and Demand characteristics are less of an issue.
• Weaknesses:

  Ppts can never be matched 100%- there will always ne some differences.

  Matching Pairs can be time consuming and expensive.
• Individual differences-

  All the ways which people differ from one another, especially psychological differences such as personality and intelligence.
• Order Effects-

  A confounding variable that arises as a result of the order in which the ppt completes the condition in an experience.
• Ppts could display demand characteristics as a result of taking part in the two conditions.
• Counterbalancing-

  Measure taken to control impact of order effects.
• Ppts in a repeated measure are split into two groups (A and B) ppts in group A complete condition 1 first then they complete condition 2.
• Ppts in group B complete condition 2 first and then they complete condition 1.
• Random allocation-

  Attempts to deal with individual differences.
• This is when ppts in an independent group design are randomly assigned which condition they will take part in.