An industrial organizational psychologist working for a large retail store's customer service department believes that periodic massage therapy for the service representatives might affect their customer service rating scores. For this study, 24 service representatives spent 2 months going about their normal business interacting with clients. Then for the next two months, the same 24 service representatives were given periodic massage therapy. Customer service rating scores were compared after each 2 month period. What is the Independent Variable or Grouping Variable in this