1. Does ginkgo improve memory? The law allows marketers of herbs and other natural substances to make health claims that are not supported
by evidence. Brands of ginkgo extract
claim to "improve memory and concentration." A randomized comparative experiment found no
statistically significant evidence for such effects. The subjects were 350 healthy volunteers over
60 years old. They were randomly assigned to ginkgo or a placebo pill (a dummy pill that looks
and tastes the same). All the subjects took a battery of tests for learning and memory before
treatment started and again after six weeks.

(d) Name two variables that were controlled in this experiment and why was it beneficial to
control these variables? (3 pts)