Experimental Design

Recap What are IVs and DVs?

Conditions In an experiment you can have two conditions The experimental condition is where you manipulate a variable to see the effect The control condition is where nothing is manipulated so that we have a base line to compare our results with.

Conditions Identify the conditions: An experimenter wants to see if drug X improves sport performance. One group are given the drug, another are given a placebo. Both groups are tested for athletic ability. Note that not all experiments will have a control condition (when testing for a difference between men and women for example)

Task On the handout Identify the IV and the DV Identify the two or more conditions in the experiment, highlighting which condition is the control (if there is one)

What do we mean? Experimental design refers to how we use our participants in our experiment There are three main ways that participants can be used. Independent measures Repeated measures Matched pairs

Repeated measures In repeated measures, each participants takes part in every condition E.g. to test the effect of caffeine on mood, participants have their mood assessed, drink a cup of coffee, then have their mood assessed again. Control condition: no caffeine Experimental condition: caffeine =

Repeated Measures Strengths Weaknesses No participant variables Fewer participants are needed Weaknesses Risk of demand characteristics Risk of order effects; boredom, fatigue and practice effects

≠ Independent Measures In an independent measures design, each participant is only used in one condition E.g. to test the effect of caffeine on mood, one group of participants are given coffee, another group are not. Both groups have their mood assessed before and after Control condition: no caffeine Experimental condition: caffeine ≠

Independent measures Strengths Weaknesses lower risk of demand characteristics No order effects such as boredom, fatigue, or practice effects Weaknesses Higher risk of participant variables More participants are needed

Matched pairs Matched pairs is similar to repeated measures, but the participants in the groups are matched with each other, so that the two groups are as similar as possible. E.g. before a study, the experimenter selects two groups of participants, and matches them on age, gender and intelligence. One group of participants are given coffee, another group are not. Control condition: no caffeine Experimental condition: caffeine ≠

Matched pairs Strengths Weaknesses Participant variables are reduced No order effects Less risk of demand characteristics Weaknesses Participants can never be completely matched Time consuming and expensive More participants required

Experimental designs activity I am testing the effect of music on concentration. In my experimental condition I will play loud music and get the participants to answer general knowledge questions. In my control condition I will give a general knowledge test without music

Experimental designs activity Stand up! You are my participants. Organise yourself into Independent measures Repeated measures Matched pairs

Task Identify the experimental design used in the examples