Experimental Design

What are the 3 experimental designs

• Individual Groups

• Repeated Measures

• Matched Pairs

Explain the independent groups design and its strengths and limitations

• different participants in each condition

• Strengths: low/none order effects and demand characteristics

• Limitations: individual differences between participants

  • Can control with random allocation to conditions

Explain the repeated measures design and its strengths and limitations

• participants take part in all conditions

• Strengths: no individual differences, more economical

• Limitations: order effects

  • Can control with counterbalancing or randomisation

Explain the matched pairs design and its strengths and limitations

• participants are matched with someone similar in other condition(s) on an individual difference e.g. IQ

• Strengths: Less/none order effects, demand characteristics, individual differences

• Limitations: not economical

What are individual differences

Participants differ in many ways

Which may interfere with effect of the IV on DV

What are order effects

• Events in experiments occur in an order

• So earlier events may affect later events

• E.g. fatigue or practice

Explain how a researcher would implement random allocation in an independent groups design

• each participant in the sample could be given a number

• Numbers are put in a hat / computer program

• First 50 numbers drawn from hat / program..

• would be allocated to one of the groups

Explain how a researcher would implement counterbalancing in a repeated measures design and how it controls order effects

• the sample would be randomly allocated by hat / computer program

• To take part in conditions one of two orders - A B or B A

• Order effects occur in both condition directions but cancel each other out

Explain how a researcher would implement randomisation in a repeated measures design and how it controls order effects

• Mix items together in a single sequence

• Instead of participants take part in one condition in its entirety then the other

• Participants switch between conditions continuously

• Order effects still occur in both directions but they cancel each other out

Why is randomisation rarely used

• its rare for conditions to be divisible

• Conditions usually consist of one big thing that happens

Explain how a researcher would implement a matched pairs design

• order participants on a specified variable e.g. age

• Pair each participant with their closest counterpart

• Randomly allocation from within pairs to conditions