Since muscle tension in the head region has been associated with headaches, you reason that if the muscle tension could be reduced, perhaps the headaches would decrease or go away altogether. You design an experiment in which nine subjects with tension headaches participate. The subjects keep daily logs of the number of headaches they experience during a 2-week baseline period. Then you train them to lower their muscle tension in the head region, using a biofeedback device. After six weeks of training, during which the subjects have become successful at maintaining low muscle tension, they again keep a 2-week log of the number of headaches experienced. The following are the number of headaches recorded during each 2-week period.
Subject Baseline After training