A randomized comparative experiment compared effects of two types of deep-brain stimulation (pallidal stimulation and subthalamic stimulation) on change in motor function, as blindly assessed on the Unified Parkinson's Disease Rating Scale, part III (UPDRS-III). The abstract of the study said: "Mean changes in the primary outcome did not differ significantly between the two study group (P = 0.50)." The P-value refers to a null hypothesis of "no change" in measurements between pallidal stimulation. Explain clearly why this value provides no evidence of change.