A nutritionist wants to conduct a study to validate the efficacy of an herb as an aid in weight loss. She randomly assigns half of a group of overweight persons to a treatment group who are given the herb with instructions as to use and a planned diet for six weeks. The other half of the group is given the same diet without the herb. Participants are aware as to which group they belong. A nurse at the nutrition center weighs each subject on Friday of each week. Identify any problems that are likely to cause confounding.
◦ Method of assignment to treatment and control groups
◦ The study is essentially free of confounding sources.
◦ The placebo effect
◦ Experimenter effect