The appraisal of pain is entrusted to self-evaluation by patients. In the clinical research the measure of pain intensity (PI) generally refers to a whole population examined and is expressed as a mean value, balancing the up and down evaluations. In the clinical practice the pain estimate is individual and can fluctuate depending on the personality of each patient. In both cases, several clinical factors can influence the judgment of perceived pain. Psychological distress, particularly depression , sleep disturbances,  some features of pain such as neuropathic and breakthrough pain  and the nature of the disease causing pain can all vary the PI experienced.