Half of the subjects in an experiment-the experimental group-consumed large quantities of a popular artificial sweetener. Afterward, this group showed lower cognitive abilities than did the other half of the subjects-the control group who did not consume the sweetener. The detrimental effects were attributed to an amino acid that is one of the sweetener' s principal constituents.
A second experiment in which subjects consumed large quantities of the sweetener lacked a control group of subjects who were not given the sweetener would best support the conclusion that some ingredient of the sweetener was responsible for the experimental results.