shishaochen commented on issue #24470: [SPARK-27577][MLlib] Correct thresholds 
downsampled in BinaryClassificationMetrics
URL: https://github.com/apache/spark/pull/24470#issuecomment-489418353
 
 
   @srowen Get your point!
   Actually, if we choose score of the last element in each chunk as threshold, 
the calculated Recall, Precision, FMeasure on each threshold are exactly the 
same as those when no sampling (`numBins=0`).
   In other words, **they are accurate metrics**. The only difference is the 
count of thresholds when printing precision/recall/f1 curve compared to 
downsampling.
   Thus, why not return the correct metrics of full data set but approximate 
values?

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