Researchers also need to determine the number of groups needed for their study and whether a control group should be used. For example, if researchers are comparing the effects of 3 drugs, their study may need 3 groups. If the researchers would like to compare the effects of the 3 drugs with no treatment or standard treatment, a control group is also needed, or 4 groups total.
A control group, or a group that does not receive an intervention but receives standard or no treatment, is used to determine whether an effect is caused by an intervention or by some other factor, such as spontaneous remission, time, or an external influence. A control group also permits a better statistical analysis to demonstrate effectiveness.
The number of independent variables in a study, or variables that are being manipulated, will affect the number of groups needed. Typically, researchers will have 1 independent variable, such as drug dosage, and 1 moderator variable (a secondary independent variable, such as sex). A study can have 2 independent variables, such as drug type and dosage, and add the moderator variable for a 3-way design. With this type of study, however, researchers need a large total number of participants to maintain a reasonable number per group. For example, researchers conducting a study of 2 drugs would need to have 2 groups. If the researchers would like to study 2 dosages for each drug, they would then need 4 groups. If sex is added as a moderator variable, then the researchers need 8 groups. If 10 participants are needed for each group, then 80 participants are needed. Researchers may need to determine how many participants they can access to determine the number of groups that their study can support. They should enlist the help of a statistician.