To deal with the multi-level relevance, the discounted cumulative gain (DCG) (Jarvelin and Kekalainen, 2000) was adopted as the evaluation metric, defined as where i denotes the i-th document in the retrieved list; G[i] denotes the gain value, i.e., relevance levels, of the i-th document, and c denotes the parameter to penalize a retrieved document in a lower rank.
the DCG simultaneously considers the relevance levels, and the ranks in the retrieved list to measure the retrieval precision.