Experimental Design

What does experimental design refer to?

How participants are allocated to the different conditions in an experiment.

What are independent groups?

• Different participants are used for each condition of the independent variable.
  → This usually means that one group participants completes the control condition and a different group of participants completes the experimental condition.

What are strengths of independent group?

• Order effects have been removed
  → Boredom, practice and fatigue cannot act as an EV affection the results.

• Less chance of demand characteristics because participants do one condition.
  → This means they are less likely to guess the ai9m of the study.

What are limitations independent groups?

• More participants are needed as the participants in the control and experimental group differ
  → Time consuming & expensive for researcher

• Participant variables have not been controlled as there are different participants in the control & experimental group
  → Individual differences may affect the results- so we are less likely to establish a cause and effect relationship.

What is repeated measures?

Each participant completes all conditions in an experiment.
→ The same group of pps completes all the experimental & control conditions.

What are strengths of repeated measures?

• Less participants are needed, as the same participants complete all conditions. Which saves time and money for the researcher.  

• Participant variables are controlled for. This is because the same participants are in the control and experimental group which controls for individual differences. This allows for a cause and effect relationship to be established between IV and DV. 

What are limitations of repeated measures?

• Demand characteristics is more likely to occur. This is because the participants take part in both conditions. So they are more likely to guess the aim of the study.  

• Order effects will occur as the participants are completing both conditions. This means boredom, practice and fatigue will act as an EV affecting the results. 

What are matched pairs?

Pairs of participants are matched based on one key variable such as age, IQ, gender.

Participants with a similar score are paired together. One person from each pair, will be randomly allocated to the control group and the other to the experimental group.  

So the participants take part in one condition only. 

What are strengths of matched pairs?

Order effects have been removed. This means boredom, practice and fatigue cannot act as an EV affecting the results.  

Less chance of demand characteristics because participants only do one condition. This means they are less likely to guess the aim of the study  

As the researcher matched the participants on a key variable. This allows for the control of one participant variable. 

What are participant variables?

Variables that are only an issue with an independent measures design.

Participants in each group may differ. This is an issue because the difference in the DV may be due to participant variables and not due to changes in the IV.  This can act as an extraneous variable and reduce the validity of the results.

How may participant variables be controlled?

Participant variables can be dealt with by randomly allocating students to a condition.

This can be achieved through randomisation: all names are put into a hat. Randomly select participants to go into the control group and the experimental group.

This is done so the researcher does not influence who goes in which group. It also means that, chances are, participant variables will be evened out between conditions.

What are order effects?

• Order effects only occur in a repeated measures design

• Order effects refer to the order of the conditions having an effect on the participants’ behaviour.  Performance in the second condition may be better because the participants know what to do . Or there performance might be worse in the second condition because they are tired.

How could we control order effects?

•To combat order affects the research counter balances the order of the conditions for the participants.
 Alternating the order in which participants perform in different conditions of an experiment. 
The sample is split in two groups experimental (A) and control (B). 

•For example, group 1 does ‘A’ then ‘B’, group 2 does ‘B’ then ‘A’ this is to eliminate order effects.

• Although order effects occur for each participant, because they occur equally in both groups, they balance each other out in the results.