Swedish researchers concluded that viewing and discussing the arts soothes the soul and helps relieve medical conditions such as high blood pressure. This conclusion was based on a study in which 100 random selected elderly women gathered once a week to discuss different works of art. The study also included a control group of
100 random selected elderly women who met once a week to discuss their hobbies and interests. At the end of 4 months, the art discussion group was found to have a
more positive mental attitude and to have lower blood pressure than the control group. What is the explanatory variable for this study: