Placebo vs Control Group
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.
Placebo
Collecting data and study design
A placebo is a dummy treatment with no active ingredient, given so a group's experience matches the real treatment except for the component being tested.
A placebo controls for the placebo effect, the tendency of people to respond simply because they believe they are being treated. A sugar pill, saline injection, or sham procedure looks and feels like the real thing but does nothing on its own. For example, in a drug trial the control group takes an identical-looking sugar pill so both groups share the same expectations. Any difference in outcomes between the groups can then be credited to the drug's active ingredient rather than to belief.
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.