Data Transformation - Sample and Split

This article describes the modules in Azure Machine Learning Studio that you can use to partition or sample data.


Applies to: Machine Learning Studio

This content pertains only to Studio. Similar drag and drop modules have been added to the visual interface in Machine Learning service. Learn more in this article comparing the two versions.

Splitting and sampling datasets are both important tasks in machine learning. For example, it's a common practice to divide data into training and testing sets to help you evaluate a model on a holdout dataset. Sampling is also increasingly important in the era of big data, to ensure that there's a fair distribution of classes in your training data. Sampling also helps ensure that you're not processing more data than is needed.

You can use Machine Learning Studio modules to customize the way you split or sample datasets:

  • Filter training data based on an attribute in the data.
  • Perform stratified sampling to divide the class variable equally among n number of groups.
  • Divide source data into a training and testing data set by using a custom ratio.
  • Apply regular expressions to the data to filter out invalid values.

Choosing the right operation: Split or sampling

Machine Learning Studio provides two modules that encapsulate tasks. The modules sound similar, but they have different uses, and provide complementary functionality. It's likely that you'll use both modules in an experiment, to get the right amount and the right mix of data.

Next, we compare the Split Data module and the Partition and Sample module by seeing which tasks each module is commonly used for.

Uses of the Split Data module

  • Divide data into two groups. Use the Split Data module. The module produces exactly two splits of the data. You can specify the condition on which the data is split, and the proportion of the data to put into each subset. Split Data always saves the subset of data that doesn’t meet the conditions.
  • Allocate label values equally to datasets. The option to stratify on a specified column is supported by both modules. However, if you want to create two datasets and are mostly interested in the label column, the Split Data module is a quick solution.

Example of using the Split Data module

Suppose you imported a very large dataset from a CSV file. The dataset contains customer demographics. You want to create different models for customers in different countries, so you decide to split the data by using the value of the Country-Region column. Here are the steps you take to complete this task:

  1. Add the Split Data module, and then specify an expression on the Country-Region field. The remainder of the data is available on the secondary output.
  2. Add another instance of the Split Data module.
  3. Repeat steps 1 and 2. Specify a different country in the expression for each iteration.

The Split Data module supports both regular expressions, for text data, and relative expressions, for numeric data.

The Split Data module also provides sophisticated functionality that you can use to divide specialized datasets. Use the functionality to create recommendation models, and to generate predictions.

Uses of the Partition and Sample module

  • Sampling. Always use the Partition and Sample module. The module provides multiple customizable sampling methods, including several options for stratified sampling.
  • Assign cases to multiple groups. Use the Assign to Fold or Pick Fold options in the Partition and Sample module.
  • Return only a subset of the data. Use the Partition and Sample module. The module gives you the specified subset on the primary output. The remaining data is available on a secondary output.
  • Get only the top 2,000 rows of a dataset. Use the Partition and Sample module. Select the Head option. This is particularly handy when you are testing a new experiment and want to run short trials of a workflow.

Example of using the Partition and Sample module

The Partition and Sample module can generate multiple partitions of the data, not just two. At the same time, it can perform various sampling operations.

For example, suppose you need to get just 10 percent of your data, while ensuring that the distribution of the target attribute is the same as in the source data. Here are the steps you take to complete this task:

  1. Add the Partition and Sample module.
  2. Choose the Sampling mode, and then specify 10%.
  3. Select the stratified sampling option, and then pick the column that contains the target attribute.

If you don't need to keep all the data, use the Partition and Sample module. The remaining data is still present in the workspace, but it doesn't need to be processed further as part of the experiment.

List of modules

This category includes the following modules:

  • Partition and Sample: Creates multiple partitions of a dataset based on sampling.
  • Split Data: Partitions the rows of a dataset into two distinct sets.

See also