Github user srowen commented on a diff in the pull request:
https://github.com/apache/spark/pull/22090#discussion_r210772634
--- 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}
--- End diff --
We do need to fix this, but, this makes the subscripts incorrect for
R_i(j). I think the expression should change to ln(j+2) in the next line; this
is what the code does. For consistency I'd do the same below too.
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