A coach in a large high school thinks that ballet training will improve the batting performance of his baseball team. He decides to have a randomly selected half of the team take six weeks of ballet training before the baseball season begins, while the other half does not take such training. He will then compare the season batting averages of group A (those with ballet training) and group B (those without ballet training) by comparing the mean of group A with the mean of group B. An independent variable is: