Examine the fictional experimental study described below:
You are studying for an exam and your roommate tells you that that ginkgo biloba (an extract made from an Asian tree) will enhance your memory. You perform an internet search to see if it really works and find a report of a study that the US Food and Drug Administration (FDA) set up to determine whether large doses of ginkgo biloba would increase a person’s memory.
In the study, five thousand volunteers were split into two groups. For two weeks, members of Group A took 120 mg of ginkgo biloba daily. Group B received 120 mg of a placebo. At the end of the two-week period, the researchers administered both a short-term memory test and a long-term memory test.
The short-term memory test consisted of flashing letters on a screen for a few seconds, then asking the subjects what those letters were several minutes later. For the long-term memory test, researchers had the subjects study a list of words and then the next day, the subjects were asked to recall as many of the words as they could. The results showed no statistically significant difference in the long-term memories of the two groups tested, but there was a slight decrease in the short-term memories of those taking the gingko biloba.
What is the independent variable in this experiment?