Experimental design refers to how participants are allocated to each condition of the independent variable, such as a control or experimental group

• Independent design (between-groups design): each participant is selected for only one group. With the independent design, the most common way of deciding which participants go into which group is by means of randomization

• Matched participants design: each participant is selected for only one group, but the participants in the two groups are matched for some relevant factor or factors (e.g. ability; sex; age)

• Repeated measures design (within groups): each participant appears in both groups so that there are
  exactly the same participants in each group

  • The main problem with the repeated measures design is that there may well be order effects. Their experiences during the experiment may change the participants in various ways

  • They may perform better when they appear in the second group because they have gained useful information about the experiment or about the task. On the other hand, they may perform less well on the second occasion because of tiredness or boredom

• Counterbalancing is the best way of preventing order effects from disrupting the findings of an experiment and involves ensuring that each condition is equally likely to be used first and second by the participants

If we wish to compare two groups with respect to a given independent variable, it is essential to make sure that the two groups do not differ in any other important way