If the same individual participates in both study and experimental group (patient serves as his or her own control), it is called:

Ans. A. Cross over studyExplanationCross over type of study designs are the randomized controlled trials, in which each patient serves as his own control. A better understanding can be taken from the diagram.Here the patient is initially either enrolled into intervention group or control group, after a specified duration the person is crossed over alternate group (e.g. patient initially enrolled in intervention arm, later after a period (say 3 months) he is crossed to control arm, i.e. he now receives control treatment, e.g. placebo).They have added advantage that all the participants receive the intervention at some time. The number of patients and time are also reduced.Cannot be used when:Drug cures the diseaseDrug is effective only in certain stage of diseaseDisease changes radically during the period of time required for the study.