Researchers were interested in whether relaxation training decreases the number of headaches a person experience.

Researchers were interested in whether relaxation training decreases the number of headaches  a person experience.  They randomly assigned 20 participants to a control group (no relaxation training) or a relaxation training group and noted the change in the number of headaches each group reported from the week before training to the week after training.  What is the independent variable in this study?  Why do you think so?