Note
Click here to download the full example code
MNIST Dataset oversampling
The example makes use of openml's MNIST dataset.
# Authors: Joao Fonseca <jpmrfonseca@gmail.com>
# Georgios Douzas <gdouzas@icloud.com>
# Licence: MIT
from collections import Counter
import matplotlib.pyplot as plt
import numpy as np
import pandas as pd
from imblearn.datasets import make_imbalance
from imblearn.over_sampling import SMOTE
from imblearn.pipeline import Pipeline
from sklearn.datasets import fetch_openml
from sklearn.ensemble import RandomForestClassifier
from sklearn.model_selection import train_test_split
from imblearn_extra.gsmote import GeometricSMOTE
def plot_mnist_samples(X, y, title=None, n_subplots=None):
if n_subplots is None:
n_subplots = [1, np.unique(y).shape[0]]
imshape = int(np.sqrt(X.shape[-1]))
fig, axes = plt.subplots(nrows=n_subplots[0], ncols=n_subplots[1], figsize=(20, 3))
if title is not None:
fig.suptitle(title, fontsize=16)
for i, val in enumerate(np.unique(y)):
images = X[y == val]
img = images.iloc[np.random.randint(images.shape[0])]
if len(np.unique(y)) > 1:
axes[i].imshow(np.reshape(img.values, (imshape, imshape)), cmap='gray')
axes[i].set_title(str(val))
axes[i].axis('off')
else:
axes.imshow(np.reshape(img.values, (imshape, imshape)), cmap='gray')
axes.set_title(str(val))
axes.axis('off')
def fit_pipelines(X_train, y_train, X_test, y_test):
pipelines = {
'No oversampling': Pipeline([('none', None), ('rfc', RandomForestClassifier(random_state=1))]),
'SMOTE': Pipeline(
[
('smote', SMOTE(random_state=34, k_neighbors=3)),
('rfc', RandomForestClassifier(random_state=1)),
],
),
'GSMOTE': Pipeline(
[
('gsmote', GeometricSMOTE(random_state=12, k_neighbors=3)),
('rfc', RandomForestClassifier(random_state=1)),
],
),
}
results = {}
for name, estimator in pipelines.items():
estimator.fit(X_train, y_train)
results[name] = estimator.score(X_test, y_test)
return pd.DataFrame(data=results.values(), index=results.keys(), columns=['Score'])
MNIST Dataset
The MNIST database is composed of handwritten digits with 784 features, the raw data is available at: http://yann.lecun.com/exdb/mnist/.
It is a subset of a larger set available from NIST. The digits have been size-normalized and centered in a fixed-size image. It is a good database for people who want to try learning techniques and pattern recognition methods on real-world data while spending minimal efforts on preprocessing and formatting. The original black and white (bilevel) images from NIST were size normalized to fit in a 20x20 pixel box while preserving their aspect ratio. The resulting images contain grey levels as a result of the anti-aliasing technique used by the normalization algorithm. the images were centered in a 28x28 image by computing the center of mass of the pixels, and translating the image so as to position this point at the center of the 28x28 field.
The function sklearn.datasets.fetch_openml will load the MNIST
dataset. It returns a tuple object with the feature matrix as the
first item and the target values in the second. The dataset will be
downloaded from the web if necessary. Afterwards we select only 1's and 7's
from the dataset, create a balanced hold-out set and use the function
imblearn.datasets.make_imbalance to make the dataset imbalanced.
X, y = fetch_openml('mnist_784', version=1, return_X_y=True, parser='auto')
selection = y.isin(['1', '7'])
X, y = X[selection], y[selection]
X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.1)
indices = np.random.RandomState(3).choice(X_train.index, 1000)
X_train, y_train = X_train.loc[indices], y_train.loc[indices]
X_train, y_train = make_imbalance(
X=X_train,
y=y_train,
sampling_strategy={'1': 5},
random_state=9,
)
X_test, y_test = make_imbalance(
X=X_test,
y=y_test,
sampling_strategy={'1': 100, '7': 100},
random_state=11,
)
counter_train = Counter(y_train)
counter_test = Counter(y_test)
distribution = pd.DataFrame(
{'1': [counter_train['1'], counter_test['1']], '7': [counter_train['7'], counter_test['7']]},
).set_axis(['Train', 'Test'])
distribution
Below is presented a random observation from each class from the training data:
plot_mnist_samples(X_train, y_train)
Data Generation
Below is presented the generation of a new sample using the G-SMOTE algorithm for each of the three selection strategies.
for strategy in ['combined', 'majority', 'minority']:
X_res, y_res = GeometricSMOTE(
k_neighbors=3,
selection_strategy=strategy,
random_state=5,
).fit_resample(X_train, y_train)
plot_mnist_samples(X_res[len(X_train) :], y_res[len(X_train) :], f'Generated Using G-SMOTE: {strategy.title()}')
Classification
Finally we train a Random Forest Classfier algorithm and optionally use either SMOTE and or G-SMOTE oversampling methods to predict the number in each picture of this imbalanced dataset. We present the accuracy for each estimator:
fit_pipelines(X_train, y_train, X_test, y_test)
Total running time of the script: ( 0 minutes 12.751 seconds)
Download Python source code: plot_mnist_example.py