The current knowledge of the mechanisms and response of cancer pain to treatments suggest that the simple definition of pain due to cancer is not enough to define the pain construct adequately. Although pain is a subjective sensation, and therefore more difficult to assess, a number of definable features are well known to influence its response to different treatments.
The ESS was developed to provide a clinical staging system for cancer pain. This system includes known prognostic factors for the response to treatment. The system is accurate in predicting the outcome of patients with cancer pain. It may also be useful for stratification for patients prior to randomization and clinical trials, in order to secure balance of distribution in the different treatments.

Three stages are identified:
Stage 1: indicates a good prognosis
Stage 2: indicates intermediate prognosis
Stage 3: indicates poor prognosis for pain control

Patients with features such as incidental pain, neuropathic pain, tolerance to the present opioid, a past history of alcoholism, severe psychosocial distress, and cognitive impairment will put a patient in Stage 3 and, therefore, one can expect a lesser likelihood of good response to analgesic treatment.
Patients with visceral or bone/soft tissue pain, low dose of opioids, intact cognitive status, and absence of severe psychological distress are more likely to respond well to analgesic treatment.
All patients with cancer pain are staged on admission.

Bruera E. et al. A prospective multicenter assessment of the Edmonton staging system for cancer pain. JPSM;10(5):348–355.