plot_sampling_strategy_usage.py

"""
====================================================
How to use ``sampling_strategy`` in imbalanced-learn
====================================================

This example shows the different usage of the parameter ``sampling_strategy``
for the different family of samplers (i.e. over-sampling, under-sampling. or
cleaning methods).

"""

# Authors: Guillaume Lemaitre <g.lemaitre58@gmail.com>
# License: MIT

# %%
print(__doc__)
import seaborn as sns

sns.set_context("poster")

# %% [markdown]
# Create an imbalanced dataset
# ----------------------------
#
# First, we will create an imbalanced data set from a the iris data set.

# %%
from sklearn.datasets import load_iris

from imblearn.datasets import make_imbalance

iris = load_iris(as_frame=True)

sampling_strategy = {0: 10, 1: 20, 2: 47}
X, y = make_imbalance(iris.data, iris.target, sampling_strategy=sampling_strategy)

# %%
import matplotlib.pyplot as plt

fig, axs = plt.subplots(ncols=2, figsize=(10, 5))
autopct = "%.2f"
iris.target.value_counts().plot.pie(autopct=autopct, ax=axs[0])
axs[0].set_title("Original")
y.value_counts().plot.pie(autopct=autopct, ax=axs[1])
axs[1].set_title("Imbalanced")
fig.tight_layout()

# %% [markdown]
# Using ``sampling_strategy`` in resampling algorithms
# ====================================================
#
# `sampling_strategy` as a `float`
# --------------------------------
#
# `sampling_strategy` can be given a `float`. For **under-sampling
# methods**, it corresponds to the ratio :math:`\alpha_{us}` defined by
# :math:`N_{rM} = \alpha_{us} \times N_{m}` where :math:`N_{rM}` and
# :math:`N_{m}` are the number of samples in the majority class after
# resampling and the number of samples in the minority class, respectively.

# %%

# select only 2 classes since the ratio make sense in this case
binary_mask = y.isin([0, 1])
binary_y = y[binary_mask]
binary_X = X[binary_mask]

# %%
from imblearn.under_sampling import RandomUnderSampler

sampling_strategy = 0.8
rus = RandomUnderSampler(sampling_strategy=sampling_strategy)
X_res, y_res = rus.fit_resample(binary_X, binary_y)
ax = y_res.value_counts().plot.pie(autopct=autopct)
_ = ax.set_title("Under-sampling")

# %% [markdown]
# For **over-sampling methods**, it correspond to the ratio
# :math:`\alpha_{os}` defined by :math:`N_{rm} = \alpha_{os} \times N_{M}`
# where :math:`N_{rm}` and :math:`N_{M}` are the number of samples in the
# minority class after resampling and the number of samples in the majority
# class, respectively.

# %%
from imblearn.over_sampling import RandomOverSampler

ros = RandomOverSampler(sampling_strategy=sampling_strategy)
X_res, y_res = ros.fit_resample(binary_X, binary_y)
ax = y_res.value_counts().plot.pie(autopct=autopct)
_ = ax.set_title("Over-sampling")

# %% [markdown]
# `sampling_strategy` as a `str`
# -------------------------------
#
# `sampling_strategy` can be given as a string which specify the class
# targeted by the resampling. With under- and over-sampling, the number of
# samples will be equalized.
#
# Note that we are using multiple classes from now on.

# %%
sampling_strategy = "not minority"

fig, axs = plt.subplots(ncols=2, figsize=(10, 5))
rus = RandomUnderSampler(sampling_strategy=sampling_strategy)
X_res, y_res = rus.fit_resample(X, y)
y_res.value_counts().plot.pie(autopct=autopct, ax=axs[0])
axs[0].set_title("Under-sampling")

sampling_strategy = "not majority"
ros = RandomOverSampler(sampling_strategy=sampling_strategy)
X_res, y_res = ros.fit_resample(X, y)
y_res.value_counts().plot.pie(autopct=autopct, ax=axs[1])
_ = axs[1].set_title("Over-sampling")

# %% [markdown]
# With **cleaning method**, the number of samples in each class will not be
# equalized even if targeted.

# %%
from imblearn.under_sampling import TomekLinks

sampling_strategy = "not minority"
tl = TomekLinks(sampling_strategy=sampling_strategy)
X_res, y_res = tl.fit_resample(X, y)
ax = y_res.value_counts().plot.pie(autopct=autopct)
_ = ax.set_title("Cleaning")

# %% [markdown]
# `sampling_strategy` as a `dict`
# -------------------------------
#
# When `sampling_strategy` is a `dict`, the keys correspond to the targeted
# classes. The values correspond to the desired number of samples for each
# targeted class. This is working for both **under- and over-sampling**
# algorithms but not for the **cleaning algorithms**. Use a `list` instead.

# %%
fig, axs = plt.subplots(ncols=2, figsize=(10, 5))

sampling_strategy = {0: 10, 1: 15, 2: 20}
rus = RandomUnderSampler(sampling_strategy=sampling_strategy)
X_res, y_res = rus.fit_resample(X, y)
y_res.value_counts().plot.pie(autopct=autopct, ax=axs[0])
axs[0].set_title("Under-sampling")

sampling_strategy = {0: 25, 1: 35, 2: 47}
ros = RandomOverSampler(sampling_strategy=sampling_strategy)
X_res, y_res = ros.fit_resample(X, y)
y_res.value_counts().plot.pie(autopct=autopct, ax=axs[1])
_ = axs[1].set_title("Under-sampling")

# %% [markdown]
# `sampling_strategy` as a `list`
# -------------------------------
#
# When `sampling_strategy` is a `list`, the list contains the targeted
# classes. It is used only for **cleaning methods** and raise an error
# otherwise.

# %%
sampling_strategy = [0, 1, 2]
tl = TomekLinks(sampling_strategy=sampling_strategy)
X_res, y_res = tl.fit_resample(X, y)
ax = y_res.value_counts().plot.pie(autopct=autopct)
_ = ax.set_title("Cleaning")

# %% [markdown]
# `sampling_strategy` as a callable
# ---------------------------------
#
# When callable, function taking `y` and returns a `dict`. The keys
# correspond to the targeted classes. The values correspond to the desired
# number of samples for each class.


# %%
def ratio_multiplier(y):
    from collections import Counter

    multiplier = {1: 0.7, 2: 0.95}
    target_stats = Counter(y)
    for key, value in target_stats.items():
        if key in multiplier:
            target_stats[key] = int(value * multiplier[key])
    return target_stats


X_res, y_res = RandomUnderSampler(sampling_strategy=ratio_multiplier).fit_resample(X, y)
ax = y_res.value_counts().plot.pie(autopct=autopct)
ax.set_title("Under-sampling")
plt.show()