1. Introduction

1.1. API’s of imbalanced-learn samplers

The available samplers follow the scikit-learn API using the base estimator and incorporating a sampling functionality via the sample method:

Estimator:

The base object, implements a fit method to learn from data:

estimator = obj.fit(data, targets)

Resampler:

To resample a data sets, each sampler implements a fit_resample method:

data_resampled, targets_resampled = obj.fit_resample(data, targets)

Imbalanced-learn samplers accept the same inputs as scikit-learn estimators:

• data, 2-dimensional array-like structures, such as:

  • Python’s list of lists list,

  • Numpy arrays numpy.ndarray,

  • Panda dataframes pandas.DataFrame,

  • Scipy sparse matrices scipy.sparse.csr_matrix or scipy.sparse.csc_matrix;

• targets, 1-dimensional array-like structures, such as:

  • Numpy arrays numpy.ndarray,

  • Pandas series pandas.Series.

The output will be of the following type:

• data_resampled, 2-dimensional aray-like structures, such as:

  • Numpy arrays numpy.ndarray,

  • Pandas dataframes pandas.DataFrame,

  • Scipy sparse matrices scipy.sparse.csr_matrix or scipy.sparse.csc_matrix;

• targets_resampled, 1-dimensional array-like structures, such as:

  • Numpy arrays numpy.ndarray,

  • Pandas series pandas.Series.

Pandas in/out

Unlike scikit-learn, imbalanced-learn provides support for pandas in/out. Therefore providing a dataframe, will output as well a dataframe.

Sparse input

For sparse input the data is converted to the Compressed Sparse Rows representation (see scipy.sparse.csr_matrix) before being fed to the sampler. To avoid unnecessary memory copies, it is recommended to choose the CSR representation upstream.

1.2. Problem statement regarding imbalanced data sets

The learning and prediction phrases of machine learning algorithms can be impacted by the issue of imbalanced datasets. This imbalance refers to the difference in the number of samples across different classes. We demonstrate the effect of training a Logistic Regression classifier with varying levels of class balancing by adjusting their weights.

As expected, the decision function of the Logistic Regression classifier varies significantly depending on how imbalanced the data is. With a greater imbalance ratio, the decision function tends to favour the class with the larger number of samples, usually referred to as the majority class.