One of the best experimental studies is the diabetes prevention program which started two years ago.  In this experimental study, the subjects were randomly selected from a community and divided to do one of the three trials, ie., exercise, diet or medication.  Those high risk subjects assigned to each of the regiment were ask to followed the researcher instruction and the outcome was measured after the study period.  The limitations of this time study was the high drop out rates, the high experimental cost, and the question of ethics.