For a given ordered list of scores, this measure is based on the difference between the original order, and the order when the list is sorted by score.
if items are ranked optimally in descending order of score at position N , nDCG-N is equal to 1. nDCG is a normalised score, and indicates how close the candidate label ranking is to the optimal ranking within the set of annotated candidates, noting that an nDCG-N score of 1 tells us nothing about absolute values of the candidates.