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https://issues.apache.org/jira/browse/SPARK-17906?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15571946#comment-15571946
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Sean Owen commented on SPARK-17906:
-----------------------------------

Are you describing what the old MulticlassMetrics class supported with 
fMeasure? it would compute it for one label vs the others. I think it makes 
some sense, just a question of how to expose it meaningfully and in a backwards 
compatible way. 

> MulticlassClassificationEvaluator support target label
> ------------------------------------------------------
>
>                 Key: SPARK-17906
>                 URL: https://issues.apache.org/jira/browse/SPARK-17906
>             Project: Spark
>          Issue Type: Brainstorming
>          Components: ML
>            Reporter: zhengruifeng
>            Priority: Minor
>
> In practice, I sometime only focus on metric of one special label.
> For example, in CTR prediction, I usually only mind F1 of positive class.
> In sklearn, this is supported:
> {code}
> >>> from sklearn.metrics import classification_report
> >>> y_true = [0, 1, 2, 2, 2]
> >>> y_pred = [0, 0, 2, 2, 1]
> >>> target_names = ['class 0', 'class 1', 'class 2']
> >>> print(classification_report(y_true, y_pred, target_names=target_names))
>              precision    recall  f1-score   support
>     class 0       0.50      1.00      0.67         1
>     class 1       0.00      0.00      0.00         1
>     class 2       1.00      0.67      0.80         3
> avg / total       0.70      0.60      0.61         5
> {code}
> Now, ml only support `weightedXXX`. So I think there may be a point to 
> improve.
> The API may be designed like this:
> {code}
> val dataset = ...
> val evaluator = new MulticlassClassificationEvaluator
> evaluator.setMetricName("f1")
> evaluator.evaluate(dataset)       // weightedF1 of all classes
> evaluator.setTarget(0.0).setMetricName("f1")
> evaluator.evaluate(dataset)       // F1 of class "0"
> {code}
> what's your opinion? [~yanboliang][~josephkb][~sethah][~srowen] 
> If this is useful and acceptable, I'm happy to work on this. 



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