Experimental Design

• control condition


  a condition within an experiment where the participants are not exposed to the intervention or manipulation of the IV
• experimental condition


  a condition within an experiment where the participants are exposed to the intervention or manipulation of the IV
• experimental design


  The way we organise the control and experimental conditions is called experimental designwe need to be cautious as a potential weakness in experimental design can undermine the validity of our research
• the 3 main features of experimental design

  • repeated measures-independent groups-matched pairs.
• strengths and weaknesses associated with experimental design involve these 4 features

  order effects

  demand characteristics

  the number of participants

  participant variables
• order effects


  Order effects occur when participating in one condition may affect how a participant performs in another.Participants performance in the experimental condition may be better, not because of the IV being manipulated but because of practice effects after completing the control condition.On the other hand, their performance in the experimental condition may be worse, again not because of the IV being manipulated but because of fatigue or the boredom.
• demand characteristics


  Demand characteristics are a type of confounding variable where participants unconsciously work out the aim and act differently (either through social desirability or the screw you effect). If a participant takes part in more than one condition, they are more likely to be able to work out the aim of the experiment
• the number of participants


  The number of participants required to collect a sufficient amount of data depends on what sort of experimental design you use. Using fewer participants is a practical choice in research and can save a researcher a great deal of time and effort.
• participant variables


  Participant variables are the individual characteristics that all participants have. Participant variables such as age, gender, personality, intelligence or education background could affect how we respond in a piece of research.
• repeated measures


  -an experimental design where participants take part in both the control condition and the experimental condition-it is perhaps the most straightforward experimental design-the performance of the participants on the control condition is compared to their performance on the experimental condition. If there is a significant difference in the performance of the participants in the two conditions, we can say that the IV has caused an effect on the DV.
• strengths of repeated measures

  the number of participants- repeated measures uses fewer participants than either independent groups or matched pairsparticipant variables- they are not a problem as a participants performance is being measured against themselves and not another person
• weaknesses of repeated measures

  order effects- can be a problem is counterbalancing has not been conducteddemand characteristics- more likely to occur as the participant has been exposed to more than one condition
• independent groups


  -an experimental design where participants take part in only one experimental condition-it is a straightforward experimental design-A researcher might use an independent groups design by randomly allocating participants in his sample group to take part in either the control condition or the experimental condition, but not both (as with repeated measures).
• strengths of independent groups

  order effects- there are no order effects with independent groups as participants only take part in one conditiondemand characteristics- there is less chance of demand characteristics than with repeated measures as participants only take part in one condition
• weaknesses of independent groups

  participant variables- there may be unknown participant variables between the groups of participants in the control and experimental conditions that are responsible for any type of difference in the DVnumber of participants- an independent groups design needs twice as many participants as a repeated measures design to produce the same amount of data
• Matched pairs

  A form of independent groups design where the experimental and control participants are deliberately similar
• strengths of matched pairs

  number of participants- matched pairs require twice as many participants as independent groupsparticipant variables- matched pairs design is less likely to be affected by participant variables than independent groups
• Matched pairs design

  • Probably the most complex of the experimental designs
  • Instead of just randomly allocating participants into either the experimental or control conditions, the researcher uses pairs of participants who have been matched for any participant variable that the researcher believes may influence performance on the DV
• Matched pairs design can be quite costly in terms of time and money, but it is one of the few ways to deal with the problem of participant variables that may occur in an independent groups design
• weaknesses of matched pairs

  order effects- less likely to be a problem as participants only take part in one conditiondemand characteristics- matching pairs of participants is a costly and difficult task