As you are all very fully aware, we’ve spent the last 2 weeks doing about t-tests, which has been positively riveting. We’ve looked at between subjects t-tests and within groups t-tests, and so I thought I’d base my first blog of 2012 on the two different designs and which is better. Now try not to explode with excitement.
Firstly I’ll talk about the independent measures design. This is when each condition uses different participants. Because of this, our results don’t suffer from order effects, such as fatigue and boredom.
However, this design can lead to validity problems such as maturation (the effects of a treatment, for example), the participant gets used to being tested, or participants on one condition may talk to participants on another about the experiment.
The biggest downfall of IMD is that it suffers greatly from individual differences. No two people are the same; some people may find the task(s) in the experiment easy, and some may find it more difficult. For example, if it was a memory task, someone with poor memory would struggle more than someone with a reasonably good memory. Also, if the person tested is a psychology student, they may be able to figure out what the researcher is testing for, as opposed to someone who has not studied psychology, and so this could lead to demand characteristics.
So IMD doesn’t sound all that great, so let’s look at the alternative. Repeated measures design uses the same participants in all conditions, and so this removes individual differences as a potential confounding variable. Also, because the same participants are used in all the conditions then this means fewer participants are required, which can save on time and money.
However, RMD has its fair share of problems; it may not be possible to test all participants twice or more. Unlike IMD, because participants are in each condition, they could start to suffer from the order effects that IMD manages to avoid. But this is not the end of the world; these can be minimised using counterbalancing.
These order effects occur when people behave differently because of the order of the conditions, for example, performance may be enhanced by a practice effect, or it could be made worse from fatigue and boredom. Counterbalancing consists of some participants doing the conditions in one order, and others doing them in a different order. This randomises the order effects.
After looking at both designs, it is clear to see that repeated measures design comes out on top. Although both designs have their strengths and weaknesses, it is only repeated measures design that can counter their biggest weakness.