Scientists conduct a study in which 100 adults with chronic diarrhea are divided into 2 groups of 50 people. one group is given a supply of yogurt that contains a certain kind of bacteria and the other group is given yogurt that is bacteria free. the study's participants and researchers do not know which group of subjects has the bacteria in the yogurt and which group does not. the scientists instruct the participants to eat the entire 8 ounces of yogurt once a day for a month and record the their bowel habits. this is an example of a(n)