srowen commented on a change in pull request #23589: [SPARK-26351][mllib]Update
doc and minor correction in the mllib evaluation metrics
URL: https://github.com/apache/spark/pull/23589#discussion_r249247623
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File path: docs/mllib-evaluation-metrics.md
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@@ -462,10 +462,10 @@ $$rel_D(r) = \begin{cases}1 & \text{if $r \in D$}, \\ 0
& \text{otherwise}.\end{
<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}
- \frac{rel_{D_i}(R_i(j))}{\text{ln}(j+2)}} \\
+ \frac{rel_{D_i}(R_i(j))}{log_2(j+2)}} \\
Review comment:
The base of the log doesn't matter as it will put a constant multiplier on
every term in the ratio. Internally it's computed as `ln` as you note below. We
could just write `log`, even, to indicate it doesn't matter.
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