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From jkbradley <...@git.apache.org>
Subject [GitHub] spark pull request: [SPARK-9910][ML]User guide for train validatio...
Date Wed, 26 Aug 2015 00:54:08 GMT
Github user jkbradley commented on a diff in the pull request:

    https://github.com/apache/spark/pull/8377#discussion_r37937327
  
    --- Diff: docs/ml-guide.md ---
    @@ -801,6 +801,173 @@ jsc.stop();
     
     </div>
     
    +## Example: Model Selection via Train Validation Split
    +In addition to  `CrossValidator` Spark also offers
    +[`TrainValidationSplit`](api/scala/index.html#org.apache.spark.ml.tuning.TrainValidationSplit)
for hyper-parameter tuning.
    +It randomly splits the input dataset into train and validation sets based on ratio passed
as parameter
    +and use evaluation metric on the validation set to select the best model.
    +The use is similar to `CrossValidator`, but simpler and less computationally expensive.
    +
    +`TrainValidationSplit` takes an `Estimator`, a set of `ParamMap`s, and an
    +[`Evaluator`](api/scala/index.html#org.apache.spark.ml.Evaluator).
    +It begins by splitting the dataset into two parts using *trainRatio* parameter
    +which are used as separate training and test datasets. For example with `$trainRatio=0.75$`
(default),
    +`TrainValidationSplit` will generate training and test dataset pair where 75% of the
data is used for training and 25% for validation.
    +Similarly to `CrossValidator`, `TrainValidationSplit` also iterates through the set of
`ParamMap`s.
    +For each combination of parameters, it trains the given `Estimator` and evaluates it
using the given `Evaluator`.
    +The `ParamMap` which produces the best evaluation metric is selected as the best option.
    +`TrainValidationSplit` finally fits the `Estimator` using the best `ParamMap` and the
entire dataset.
    +
    +`TrainValidationSplit` only evaluates each combination of parameters once as opposed
to k times in
    + case of `CrossValidator`. It is therefore less expensive, but will not produce as reliable
results.
    +
    +<div class="codetabs">
    +
    +<div data-lang="scala">
    --- End diff --
    
    Use this instead:
    ```
    <div data-lang="scala" markdown="1">
    ```
    That helps when, e.g., you put links inside the code tabs (which will likely be done in
the future as docs expand).


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