In any experiment there are several levels of the independent variable (IV). For example, in Loftus and Palmer (1974) participants were given a sentence with one of the fiver verbs (hit, smashed, bumped, etc), so there were five levls of the IV. Experimenters have a choice - either each participant is tested on all of the IVs (repeated measures) or a third possibility - the participants in each independent group can be matched with participants in the other group(s) on key variables such as age and IQ (matched pairs design).
Repeated measures The same participants take part in every condition being tested. For example, if you were testing the effect of temperature on ability to work, each participant would do a task in a warm environment and again, later, in a cold environment. This means there is good control for participant variables and fewer participants needed. But order effects may occur (e.g. boredom, practice) or participants may guess the purpose of the experiment and their behaviour would be affected.
Independent groups Participants take part in only one condition. For example, some participants do the task in the warm environemtn, and some do it in the cold environment and their performance is compared. This technique avoids problems that occur with repeated measures design but needs more participants. There is no control of participant variables, although this can partly be overcome using random allocation of participants to conditions.
Matched pairs Participants matched on key particpant variables. This technique avoids order effects and partially controls participant variables but matching is difficult.
Experimenter control We have discussed the importance of controls in experimental research - to control extraneous/confounding variables. However, researchers also use the word 'control' in a different context - as a means of establishing a baseline in an experiment. You may recall seeing the words experimental group and control group. For example: A researcher might want to investigate the effects that rewards have on performance. To do this children are asked to collect rubbish from a playground and offered a chocolate bar as a reward. They collect ten bags of rubbish. We cannot conclude anything about the ffects of the reward because we don't know if ten…