Experimental design

What is an independent groups design?

An experimental design where each participant only takes part in one condition of the independent variable (IV).

What are the strengths of independent groups designs?

Strengths:

• Order effects cannot be observed, as participants are in only one condition.

• Data collection is less time-consuming if all conditions can be conducted simultaneously.

What is a repeated measures design?

An experimental design where each participant takes part in both/all conditions of the independent variable.

What are the strengths of repeated measures designs?

Strengths:

• Participant variables are controlled, since the same participants are used.

• Fewer participants are needed, as each one participates in multiple conditions.

What is a matched pairs design?

An experimental design where pairs of participants are matched on important characteristics, and one member of each pair is allocated to each condition of the IV.

What are the strengths of matched pairs designs?

Strengths:

• Order effects cannot be observed, as participants are in only one condition.

• Matching reduces the risk of participant variables affecting results.

What are the weaknesses of matched pairs design?

Weaknesses:

• Different participants must be recruited for each condition, which is difficult and expensive.

• Matching participants is complex, and perfect matches are nearly impossible.

How do you decide which experimental design to use?

• Use independent groups if order effects are a concern and participants can’t be reused.

• Use repeated measures if controlling participant variables is critical and order effects can be counterbalanced.

• Use matched pairs if you want to control participant variables but cannot use repeated measures.

what are the weaknesses of repeated measures design?

Weaknesses:

• Order effects (practice/fatigue/demand characteristics) may occur.

  • Can be reduced with counterbalancing (e.g., half do A→B, half do B→A).

• If a participant drops out, all their data is lost from all conditions.

What are the weaknesses of independent group design?

Weaknesses:

• Different participants must be recruited for each condition, which can be difficult and expensive.

• Participant variables (individual differences) may affect results, rather than manipulation of the IV.