单项选择题

The popular dietary supplement ginseng is claimed to improve one’s mood and allaround vigor, but a new study published in the Journal of the American Dietetic Association suggests that ginseng has little of any effect on psychological health.
The study, conducted by researchers at Oregon State University and Wayne State University, is one of the most extensive peer-reviewed studies of ginseng ever conducted.
"Ginseng is being marketed to relatively healthy young people as a way to feel even better—a kind of yuppie supplement," said Bradley J. Cardinal, an associate professor in the College of Health and Human Performance at Oregon State. "We found it had no real effect on mood at all. It certainly did not live up to some ofits over-enthusiastic marketing claims."
Among the claims, the authors say, was that ginseng enhances mood, leads to positive well-being, and generally makes you feel better. Marketing ploys (策略) used to push ginseng promoted its use by astronauts and professional athletes.
The study by Crdinal and Hermann J. Engels of Wayne State University focused only on the alleged psychological properties of ginseng. The researchers gave a regular, 200mg daily dose of ginseng to one group of volunteers for eight weeks. A second group received a double dose of 400mg daily; the third group received a sugar pill. None of the individuals knew what they were taking.
At the end of the eight-week period, the researchers measured the effects of the supplements on the volunteers "total mood disturbance" using a 65-question "Profile of Mood States" inventory.
To eliminate bias, the researchers evaluated the tests without initially knowing which subjects were taking ginseng and which were taking placebos. They compared the results with a baseline survey of the volunteers taken just prior to the study. They found no significant difference among the three groups.
In this latest research, the group taking 200 mg of ginseng experienced a 2.5 percent increase in positive feelings during the eight weeks, but the group taking a placebo had a greater increase, 5.0 percent. The largest gain in positive feelings, 7.7 percent, went to the group taking 400mg of ginseng, but all of those numbers were within a statistical margin of error, making the differences insignificant, the researchers say.
Cardinal said the most important category was Total Mood Disturbance, because it looked at the broad spectrum of mood enhancement experienced by the volunteers. The 200mg group experienced a greater increase, 1.8 percent, than the 400mg group, which increased only 0.3 percent. The placebo group was in between with a 1 percent increase.
"Statistically, there really was no difference between the groups that took ginseng, and the group that didn’t," Cardinal said. "It is still possible that ginseng may have an effect on certain individuals, or certain populations, such as the sick and the elderly. But higher doses also may bring on unwanted side effects. These are issues that need to be clarified with additional well controlled studies in the future."
Cardinal says their study used more volunteers (83) for a longer duration than almost all the other studies, and they even sent their ginseng to an independent laboratory to ensure that it was of high quality.
Their double-blind, placebo-controlled study was designed to eliminate bias by both the volunteers and the researchers themselves. "The bottom line," Cardinal said, "is that ginseng doesn’t seem to do much to enhance the psychological well-being of normal, healthy adults." Which of the following is NOT a measure taken to eliminate the bias