3. experimental design

what is experimental design

the different ways in which participants can be organised in relation to the experimental conditions

what are the experimental designs

1. independent groups

2. repeated measures

3. matched pairs

independent groups

• participants are allocated to different groups where each group represents one experimental condition

advantages

• order effects are not a problem

• participants are less likely to guess the aim

disadvantages

• participants are not the same regarding participant variables which means these can become a CV affecting the DV which reduces the validity

• less economical as each participant contributes a single result only which increases the time/money spent

how can participant variables be controlled

random allocation - attempts to evenly distribute participant characteristics across the conditions of the experiment using random techniques

repeated measures

all participants take part in all conditions of the experiment

advantages

• participant variables are controlled - higher validity

• fewer participants are needed so less time is spend recruiting them - more economical

disadvantages

• order of tasks done may be significant

• order effects arise as the tasks can create boredom or fatigue that may cause deterioration in performance on the second task, alternatively it may improve through the effects of practice - serve as a CV

• more likely to guess the aim leading to demand characteristics

how can order effects be controlled

counterbalancing - half the participants experience the conditions in one order, and the other half in the opposite order

• as with random allocation, it does not remove or prevent the problem, but attempts to balance out the effects

matched pairs

1. pairs of participants are first matched on some variable(s) that may affect the DV

2. one participant from each pair is randomly allocated to a different condition

advantages

• order effects and demand characteristics are less of a problem

• more control for the CV of participant variables

disadvantages

• participants can never be matched exactly - even when identical twins are used there will still be important differences that may affect the DV

• time consuming and expensive, particularly if a pre test is required - less economical