Experimental design (OCR A-Level Psychology): Revision Notes

7.2.5 Experimental design

Experimental Design

Experimental design refers to the way participants are used in experiments – how the testing is arranged with different experimental conditions.

Experimental design	Evaluation
Independent Groups – when two separate groups of participants experience two different conditions of the experiment. Assesses performances of two different groups of people	Independent Groups – The biggest issue with independent groups is that the participants who occupy the different groups are not the same – it assesses the performance of two different groups of people. To deal with this, researchers use random allocation. Independent designs are less economical as each participant contributes a single result. Twice as many participants are required. However, order effects and demand characteristics are less likely within this.
Repeated measures – all participants experience both conditions to see if there is a difference. Guarantees we are comparing 'like with like'.	Repeated Measures – each participant is required to complete tasks at least twice and the order of these tasks may be significant. Order effects may arise because repeating tasks could create fatigue/boredom that may cause deterioration in performance on the second task. Alternatively, performance may improve through practice, especially on a skill-based task. Order acts as a confounding variable. To deal with this, researchers use counterbalancing (half the participants experience conditions in one order whilst the other half experiences them in the opposite order). It is also more likely participants will work out the aim of the study having carried out more than one task. However, participant variables are more carefully controlled and fewer participants are needed.
Matched pairs • participants are first matched on some variables that may affect the DV then one member of the pair is assigned to condition A and the other B. Attempts to control confounding variable of participant variables.	Matched pairs – participants only take part in a single condition, so order effects and demand characteristics are less of a problem. Although there is some attempt to reduce participant variables, they can never actually be matched exactly. Matching may be more time-consuming and expensive, so this is less economical than other designs.