Preprocessing

The top-level preprocessing section specifies dataset splitting (train, validation, test), sample ratio (undersampling the minority class or oversampling the majority class) and dataset balancing.

preprocessing:
    sample_ratio: 1.0
    sample_size: null
    oversample_minority: null
    undersample_majority: null
    global_max_sequence_length: null
    split:
        type: random
        probabilities:
        - 0.7
        - 0.1
        - 0.2

Dataset Splitting

Data splitting is an important aspect of machine learning to train and evaluate machine learning models.

Ludwig supports splitting data into train, validation, and test sets, and this is configured using the top-level preprocessing section of the Ludwig config.

There is no set guideline or metric for how the data should be split; it may depend on the size of your data or the type of problem.

There are a few different methods that Ludwig uses to split data. Each of these methods is specified under the split subsection.

The following splitting methods are currently supported by Ludwig:

Random Split

By default, Ludwig will randomly split the data into train, validation, and test sets according to split probabilities, which by default are: [0.7, 0.1, 0.2].

split:
    type: random
    probabilities:
    - 0.7
    - 0.1
    - 0.2

However, you can specify different splitting probabilities if you'd like by setting the probabilities for each of the 3 datasets (so that they sum up to 1)

Fixed Split

If you have a column denoting pre-defined splits (train, validation and test) that you want to use across experiments, Ludwig supports using fixed dataset splits.

The following config is an example that would perform fixed splitting using a column named split in the dataset:

split:
    type: fixed
    column: split

Within the data itself, we would ensure that there is a column called split with the following values for each row in the column based on the split we want to map that row to:

• 0: train
• 1: validation
• 2: test

Note

Your dataset must contain a train split. However, the validation and test splits are encouraged, but optional.

Stratified Split

Sometimes you may want to split your data according to a particular column's distribution to maintain the same representation of this distribution across all your data subsets. This may be particularly useful when you have more than one class and your dataset is imbalanced.

In order to perform stratified splitting, you specify the name of the column you want to perform stratified splitting on and the split probabilities.

The following config is an example that would perform stratified splitting for a column color:

split:
    type: stratify
    column: color
    probabilities:
    - 0.7
    - 0.1
    - 0.2

This helps ensure that the distribution of the values in color are roughly the same across data subsets.

Datetime Split

Another common use case is splitting a column according to a datetime column where you may want to have the data split in a temporal order.

This is useful for situations like backtesting where a user wants to make sure that a model trained on historical data would have performed well on unseen future data.

If we were to use a uniformly random split strategy in these cases, then the model may not generalize well if the data distribution is subject to change over time. Splitting the training from the test data along the time dimension is one way to avoid this false sense of confidence, by showing how well the model should do on unseen data from the future.

For datetime-based splitting, we order the data by date (ascending) and then split according to the split_probabilties. For example, if split_probabilities: [0.7, 0.1, 0.2], then the earliest 70% of the data will be used for training, the middle 10% used for validation, and the last 20% used for testing.

The following config shows how to specify this type of splitting using a datetime column named created_ts:

split:
    type: datetime
    column: created_ts
    probabilities:
    - 0.7
    - 0.1
    - 0.2

Hash Split

Hash splitting deterministically assigns each sample to a split based on a hash of a provided "key" column. This is a useful alternative to random splitting when such a key is available for a couple of reasons:

• To prevent data leakage: For example, imagine you are predicting which users are likely to churn in a given month. If a user appears in both the train and test splits, then it may seem that your model is generalizing better than it actually is. In these cases, hashing on the user ID column will ensure that every sample for a user is assigned to the same split.
• To ensure consistent assignment of samples to splits as the underlying dataset evolves over time: Though random splitting is determinstic between runs due to the use of a random seed, if the underlying dataset changes (e.g., new samples are added over time), then samples may move into different splits. Hashing on a primary key will ensure that all existing samples retain their original splits as new samples are added over time.

split:
    type: hash
    column: user_id
    probabilities:
    - 0.7
    - 0.1
    - 0.2

Data Balancing

Users working with imbalanced datasets can specify an oversampling or undersampling parameter which will balance the data during preprocessing.

Warning

Dataset balancing is only supported for binary output features currently. We are working to add category support in a future release.

Note

Specifying both oversampling and undersampling parameters simultaneously is not supported.

Oversampling

In this example, Ludwig will oversample the minority class to achieve a 50% representation in the overall dataset.

preprocessing:
    oversample_minority: 0.5

Undersampling

In this example, Ludwig will undersample the majority class to achieve a 70% representation in the overall dataset.

preprocessing:
    undersample_majority: 0.7

Sample Ratio

Sometimes users may want to train on a sample of their input training data (maybe there's too much, and we only need 20%, or we want to try out ideas on a smaller subset of our data). In order to achieve this, a user can specify a sample_ratio to indicate the ratio of the dataset to use for training.

By default, the sample ratio is 1.0, so if not specified, all the data will be used for training. For example, if you only want to use 30% of my input data, you could specify a config like this:

preprocessing:
    sample_ratio: 0.3

Feature-specific preprocessing

To configure feature-specific preprocessing, please check datatype-specific documentation.