Normalized Discounted Cumulative Gain
$$
nDCG_p \overset{\text{def}}{=} \frac{DCG_p}{IDCG_p}
$$
where IDCG is the ideal discounted cumulative gain.
$$
\text{IDCG}_p \overset{\text{def}}{=} \sum_{i=1}^{|\text{REL}_p|} \frac{2^{rel_i} - 1}{\log_2(i + 1)},
$$
Normalization