Control Group vs Treatment
Both terms below come up in the same part of the course, and students mix them up. Here is each one defined on its own, side by side, so you can see where they part company.
Control group
Collecting data and study design
A control group is the set of experimental units that does not get the treatment of interest, giving a baseline to compare the treatment group against.
The control group shows what happens without the treatment, so any extra change in the treatment group can be linked to the treatment itself. It might receive nothing, a standard existing treatment, or a placebo, depending on what makes a fair comparison. For example, to test a new fertilizer you grow one set of plants with it and a control set under identical conditions without it. Comparing the two groups isolates the fertilizer's effect from sunlight, water, and soil, which act on both groups equally.
Treatment
Collecting data and study design
A treatment is a specific condition applied to subjects in an experiment, whose effect on a response variable the experiment is designed to measure.
A treatment is what the experimenter imposes on each group so their responses can be compared. For example, in a study of a new fertilizer, one treatment might be the new fertilizer and another the current fertilizer, with plant growth as the response. When an experiment varies more than one factor, each treatment is a combination of the levels of those factors. Subjects are assigned to treatments at random so the groups start out comparable.