Studies in which the exposure conditions are chosen by the investigator. For ethical reasons, exposure restrictions exist. The most important one is that examining potentially toxic substances in humans is prohibited. This implies that potentially adverse effects of food components can only be investigated in animal studies. In experimental studies, two groups of subjects are compared with regard to the outcome variable—subjects exposed to the substance under investigation (treatment group) and subjects not exposed (control group). An essential condition of this type of study is that the exposure is randomly distributed over the subjects. Maintaining all conditions constant except for the exposure has to be achieved by randomization of the study subjects, as lifestyle and genetic background differ greatly from one person to another. If possible, the study should be double-blind. This means that the investigator as well as the study subjects do not know whether they are in the treatment group or the control group. In this way, the observations are not influenced by the investigator or the respondent.
Research studies that hold the extraneous variables constant, manipulate the independent variables, and measure the dependent variables.