Despite the ability of PS to predict outcome it is a fairly blunt tool. The WHO PS is the most commonly used and is graded from 0-4. The categories are broad, for example a WHO PS of 0 could refer to a marathon runner or an elderly patient who is normally relatively inactive. Differentiating between grades is subject to observer error. There is reasonably good agreement between clinicians when differentiating between WHO grade 0-2 and 3-4, however variability exists within these subsets.