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Experimental Groups
Groups of subjects are often defined for controls and comparisons in experiments. Subjects may be assigned to different groups based on subject attributes or they may be randomly assigned to groups that will receive different treatments.
Experimental Groups are used to compare the effects of an intervention (treatment) in one population to the effects in another. For example, one might be interested in the question of whether there are differences between the memories of people diagnosed as dyslexic and "non-dyslexic" people.
If subjects are randomly assigned to groups, as in "true experiments," then effects observed in the groups will be statistically independent among groups, thereby making it easier to interpret the specific effects of different treatments and controls.
A special category of treatments is "no-treatment." Groups given no-treatments are generally called "control groups." Consequently, the major categories of experimental groups are Treatment Groups and Control Groups. In an important sense, "no treatment" is also an event because something is always going on between measurements. To demarcate the time period involved, a "Null Event" is inserted in the Control Group at the location corresponding to the location of the treatment in the Treatment Group.
In some research fields, "treatments" are called "interventions" and in others, they are called "Stimuli." Consequently, groups in which an intervention or a stimulus is presented are called "intervention groups" and "stimulus groups," respectively.
Random Assignment of Subjects to Groups at Run-time
See Random Assignment to Groups.