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https://issues.apache.org/jira/browse/SPARK-11581?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14996124#comment-14996124
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Apache Spark commented on SPARK-11581:
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User 'bharatl' has created a pull request for this issue:
https://github.com/apache/spark/pull/9560
> Example mllib code in documentation incorrectly computes MSE
> ------------------------------------------------------------
>
> Key: SPARK-11581
> URL: https://issues.apache.org/jira/browse/SPARK-11581
> Project: Spark
> Issue Type: Bug
> Components: Documentation
> Affects Versions: 1.5.1
> Reporter: Brian Webb
> Priority: Trivial
> Labels: starter
>
> The example Java code at the bottom of the mllib-decision-tree web page shows
> how to compute MSE on the test data. However, there is a bug in the code. The
> code currently divides by data.count(), but it should instead divide by the
> count of testData, testData.count().
> http://spark.apache.org/docs/latest/mllib-decision-tree.html
> Double testMSE =
> predictionAndLabel.map(new Function<Tuple2<Double, Double>, Double>() {
> @Override
> public Double call(Tuple2<Double, Double> pl) {
> Double diff = pl._1() - pl._2();
> return diff * diff;
> }
> }).reduce(new Function2<Double, Double, Double>() {
> @Override
> public Double call(Double a, Double b) {
> return a + b;
> }
> }) / data.count();
> System.out.println("Test Mean Squared Error: " + testMSE);
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