Control groups are used in independent groups designs to provide a baseline. This means that you can see whether the performance of the experimental group is higher, lower or the same as the average for the control group. Developmental psychologists use different types of control groups. For example, they may be matched on important participant variables such as age, gender, levels of cognitive, language and socio-emotional development. However, this is not always the case. This means the study may lack internal validity. The study on mindfulness, Huppert and Johnson (2010) for example, simply used five classes in the school that did not attend mindfulness classes as their control group. Other studies such as Ashdown and Bernard (2012) use a wait-list control group. Participants in such groups may or may not be carefully matched to the intervention group, however, they do receive the intervention once the main study is over. This is a strength of the study. Withholding an intervention from participants in the control group could be seen as unethical. In a wait-list study both groups ultimately can benefit from the intervention. 

Ideally, studies should use active control groups. This means that the control group undergoes some sort of intervention that is similar to the experimental group. It may have many features that are the same as the intervention being tested, such as number of sessions and may target the same areas of development. This means that any differences in the two groups at the end of the study can be attributed to specific aspects of the intervention. For example, in mindfulness studies, active control groups may take part in relaxation classes which do not attempt to increase mindfulness. This is an important consideration as the meta-analysis by Dunning et al. demonstrated. 

Key terms

Wait list control group (WLC groups) A comparison group in an experiment who does not receive the treatment/intervention until after the study has finished; greater improvement in the treatment group than the active control group cannot be explained as a simple consequence of the passing of time.

Active control group A comparison group in an experiment who receives a similar treatment/intervention to the experimental group; greater improvement in the treatment group than the active control group cannot therefore be explained by non-specific or expectancy effects alone.