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_r249256437
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File path: docs/mllib-evaluation-metrics.md
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@@ -439,21 +439,21 @@ $$rel_D(r) = \begin{cases}1 & \text{if $r \in D$}, \\ 0
& \text{otherwise}.\end{
Precision at k
</td>
<td>
- $p(k)=\frac{1}{M} \sum_{i=0}^{M-1} {\frac{1}{k}
\sum_{j=0}^{\text{min}(\left|D\right|, k) - 1} rel_{D_i}(R_i(j))}$
+ $p(k)=\frac{1}{M} \sum_{i=0}^{M-1} {\frac{1}{k}
\sum_{j=0}^{\text{min}(\left|R_i\right|, k) - 1} rel_{D_i}(R_i(j))}$
Review comment:
Maybe; it could be the same for all users, or not. The documentation above
this suggests there are equal numbers of recommended and relevant docs for each
user (Q and N) but at least, it will almost never be true that |D_i| is the
same for all users. Q could well be a constant.
But the implementation doesn't assume that and it's not necessary to, so I
might even just remove the references to Q and N, or label them "Q_i" and "N_i"
if you really want to be complete.
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