Not everyone believes that the placebo effect exists, but researchers in Houston, Texas, found evidence for it. They did a study on a common- and some interesting expensive type of knee operation. About 350.000 people a year have this surgery. The 180 volunteers in the study all understood that they might get a real knee operation or they might not. The ones who had fake surgery got only three small cuts around their knee(to make it look real). After two weeks, most patients believed that their surgery had been real. Later, 35- 40 percent of the patients who actually had surgery reported that their knees felt better.