Now, listen to part of a lecture on this topic in a psychology class.
SO，OK，let’s look at an example. A researcher wanted to study ways to prevent office workers from getting stiff and tired sitting all day at their desks. So he came up with a simple program of exercises, special stretches, and things. And he wanted to know if this exercise program would keep people from getting so tired during the work day. So he did a study. He divided the workers in an office building into two groups, group A and group B. The researcher had group A do this new exercise program, these special exercises and stretches everyday while group B they were told just to work as usual and grew a was told not to tell group b about the exercises they were doing.

The study went on for a couple of weeks. And what happened was one day some people from group A were talking about how the exercises they were doing were making them feel great that they weren’t becoming so stiff and tired during the work day. And some people from group B were sitting nearby and overheard them. So these people in group B decided to start doing the exercises too. So the researcher wasn’t able to tell how well the exercise is actually worked, because people from both groups were doing them.