People who are given a treatment may feel better (for example, their tummy may hurt less) because they believe they are getting better treatment and they expect that they will feel better. This can happen even if the treatment isn’t better. It is called a “placebo effect”. Knowing which treatment they got can also change the way people behave.
For example, someone told that eating carrots will help them lose weight may also start eating less and exercising more. So they may lose weight because of those changes, rather than because of the carrots.
REMEMBER: Ask if the people in the comparison groups knew which treatment they received and, if so, whether that may have changed how they felt or behaved.