Github user srowen commented on a diff in the pull request:
https://github.com/apache/spark/pull/22090#discussion_r210772686
--- Diff: docs/mllib-evaluation-metrics.md ---
@@ -461,11 +461,11 @@ $$rel_D(r) = \begin{cases}1 & \text{if $r \in D$}, \\
0 & \text{otherwise}.\end{
<tr>
<td>Normalized Discounted Cumulative Gain</td>
<td>
- $NDCG(k)=\frac{1}{M} \sum_{i=0}^{M-1} {\frac{1}{IDCG(D_i,
k)}\sum_{j=0}^{n-1}
+ $NDCG(k)=\frac{1}{M} \sum_{i=0}^{M-1} {\frac{1}{IDCG(D_i,
k)}\sum_{j=1}^{n}
\frac{rel_{D_i}(R_i(j))}{\text{ln}(j+1)}} \\
\text{Where} \\
\hspace{5 mm} n =
\text{min}\left(\text{max}\left(|R_i|,|D_i|\right),k\right) \\
- \hspace{5 mm} IDCG(D, k) = \sum_{j=0}^{\text{min}(\left|D\right|,
k) - 1} \frac{1}{\text{ln}(j+1)}$
+ \hspace{5 mm} IDCG(D, k) = \sum_{j=1}^{\text{min}(\left|D\right|,
k)} \frac{1}{\text{ln}(j+1)}$
</td>
<td>
<a
href="https://en.wikipedia.org/wiki/Information_retrieval#Discounted_cumulative_gain";>NDCG
at k</a> is a
--- End diff --
We can update the link here to
https://en.wikipedia.org/wiki/Discounted_cumulative_gain#Normalized_DCG
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