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https://issues.apache.org/jira/browse/MAHOUT-925?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13167573#comment-13167573
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Sean Owen commented on MAHOUT-925:
----------------------------------

This is fine, though, don't you want to count like so?

    if (numRecommendedItems > 0) {
      reach++;
    }

Otherwise it seems like you're just counting all users, except the ones that 
the *test framework* couldn't test due to sampling size, which is something 
else and something you want to ignore in general. (So actually I think I'd 
count both reach and the number of users that the test framework succeeded 
for). Is that about right for you?
                
> Evaluate the reach of recommender algorithms
> --------------------------------------------
>
>                 Key: MAHOUT-925
>                 URL: https://issues.apache.org/jira/browse/MAHOUT-925
>             Project: Mahout
>          Issue Type: Improvement
>          Components: Collaborative Filtering
>    Affects Versions: 0.5
>            Reporter: Anatoliy Kats
>            Assignee: Sean Owen
>            Priority: Minor
>         Attachments: MAHOUT-925.patch
>
>   Original Estimate: 1h
>  Remaining Estimate: 1h
>
> The evaluation of a CF algorithm should include reach, the proportion of 
> users for whom a recommendation could be made.  An algorithm usually has a 
> cutoff value on the confidence of the recommender, and if it is not high 
> enough, no recommendation is made.  The number of requested recommendations, 
> or this parameter could be varied as part of the evaluation.  The proposed 
> patch adds this.
> My build with this patch breaks 
> testMapper(org.apache.mahout.classifier.df.mapreduce.partial.Step1MapperTest):
>  org.apache.mahout.classifier.df.node.Leaf.<init>(I)V .  The test seems 
> unrelated to the patch, so I am assuming this is broken in the trunk head as 
> well.  Unfortunately I am under a deadline, and I do not have time to write 
> tests for the patch.

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