ML 105: Classification (15 pts + 10 extra)

What You Need

Purpose

To practice making simple machine learning code in Python. This project was adapted from chapter 2 of this book:

Hands-On Machine Learning with Scikit-Learn, Keras, and TensorFlow 3rd Edition, by Aurélien Géron

Using Google Colab

In a browser, go to
https://colab.research.google.com/
If you see a blue "Sign In" button at the top right, click it and log into a Google account.

From the menu, click File, "New notebook".

Getting the Data

Downloading the Data

Execute these commands to download the MNIST dataset, which contains 70,000 small images of handwritten digits. 
from sklearn.datasets import fetch_openml

mnist = fetch_openml('mnist_784', as_frame=False, parser="auto")
print(mnist.DESCR)
As shown below, a description of the data appears.

Understanding the Data

Execute these commands to put the data in variables X and y and display it in various ways: 
X, y = mnist.data, mnist.target
print(X)
print(X.shape)
print(y)
print(y.shape)
As shown below, X contains 70000 records, each with 784 values. These are the pixels of a 28x28 image.

y contains the number that was written, for all 70000 images. Notice that the first two images are '5'and '0'.

Viewing the Images

Execute these commands to see the first 100 images: 
import matplotlib.pyplot as plt

def plot_digit(image_data):
    image = image_data.reshape(28, 28)
    plt.imshow(image, cmap="binary")
    plt.axis("off")

plt.figure(figsize=(9, 9))
for idx, image_data in enumerate(X[:100]):
    plt.subplot(10, 10, idx + 1)
    plot_digit(image_data)
plt.subplots_adjust(wspace=0, hspace=0)
plt.show()
As shown below, the images are mostly recognizable, but vary a lot.

Training and Testing Sets

Execute these commands to create training and testing sets: 
X_train, X_test, y_train, y_test = X[:60000], X[60000:], y[:60000], y[60000:]
print("Training set:", len(X_train))
print("Test set:", len(X_test))
As shown below, the training set contains 60,000 images and the test set contains 10,000 images.

Preparing Binary Data

Execute these commands to create a simple dataset marking all the '5' images as 'True', and the others 'False': 
import numpy
y_train_5 = (y_train == '5')  # True for all 5s, False for all other digits
y_test_5 = (y_test == '5')

print("Training set:", y_train_5)
print("True:", numpy.count_nonzero(y_train_5 == True))
print("False:", numpy.count_nonzero(y_train_5 == False))

print()
print("Test set:", y_test_5)
print("True:", numpy.count_nonzero(y_test_5 == True))
print("False:", numpy.count_nonzero(y_test_5 == False))
As shown below, both sets are mostly 'False'.

Linear Model

We'll use a linear model -- each pixel is just multiplied by a weight factor and added together to create a single output score, which is used to determine 'True' or 'False' predictions.

The learning is a process of choosing the weights.

Training a Binary Classifier

Execute these commands to create and train a "stochastic gradient descent" (SGD) model: 
from sklearn.linear_model import SGDClassifier

sgd_clf = SGDClassifier(random_state=42, verbose=2)
sgd_clf.fit(X_train, y_train_5)

print("Prediction for image 0 (",y[0],"):", sgd_clf.predict([X[0]]))
print("Prediction for image 1 (",y[1],"):", sgd_clf.predict([X[1]]))
print("Prediction for image 2 (",y[2],"):", sgd_clf.predict([X[2]]))
As shown below, the model trains for 239 epochs, and correctly predicts the first three image values (the first one is '5' and the others are not).

Measuring Accuracy Using Cross-Validation

Cross-validation tests a model by doing these things: Of course, you could use some number of folds other than three.

Execute these commands to perform cross-validation on our model: 

from sklearn.model_selection import cross_val_score

cross_val_score(sgd_clf, X_train, y_train_5, cv=3, scoring="accuracy")
As shown below, the three accuracy measures were 95%, 96%, and 96%.

However, remember that our data set is 90% 'False' values, so that is not as good as it sounds.

Confusion Matrices

Execute these commands to calculate the "confusion matrix" -- that is, the actual number of wrong predictions from the model.

The cross_val_predict() function will perform 3-fold cross validation, and return the predicted values.

The confusion_matrix() function counts the hits and misses. 

from sklearn.model_selection import cross_val_predict

y_train_pred = cross_val_predict(sgd_clf, X_train, y_train_5, cv=3)

from sklearn.metrics import confusion_matrix

cm = confusion_matrix(y_train_5, y_train_pred)
cm
The results are in an array, outlined in the image below.

The first row contains most of the images--these are the non-5 images. The model incorrectly called 687 of them 5's.

The second row is the fives. The model incorrectly called 1891 of them non-5.

Precision

Precision measures the accuracy of the positive predictions:
Precision = (True Positives) / (True Positives + False Positives)
From the matrix in the image above: So the Precision is
Precision = 3530 / (3530 + 687) = 0.837 = 83.7%
Precision alone is not a good measure of quality, because we could make a model that is very picky, only identifying perfect matches as positive.

This model would have no False Positives, and therefore a Precision of 100%, but it would have many False Negatives.

So a second measure is needed: Recall.

Recall

Recall measures the precentage of positive instances that were correctly predicted. Recall is also called sensitivity or true positive rate.

Here's the formula for Recall:

Recall = (True Positives) / (True Positives + False Negatives)
From the matrix in the image above: So the Recall is
Recall = 3530 / (3530 + 1891) = 0.651 = 65.1%

F1 Score

The F1 score combines precision and recall into a single number called the harmonic mean:
F1 = 2 x (Precision x Recall) / (Precision + Recall)
The F1 score will only be near 1 if both Precision and Recall are. From the values above:
F1 = 2 x (0.837 x 0.651) / (0.837 + 0.651) = 0.732 = 73.2%

Calculating Precision, Recall, and F1 with sklearn

Execute these commands to calculate the Precision and Recall of your model: 
from sklearn.metrics import precision_score, recall_score, f1_score

print("Precision:", precision_score(y_train_5, y_train_pred))
print("Recall:", recall_score(y_train_5, y_train_pred))
print("F1 Score:", f1_score(y_train_5, y_train_pred))
As shown below, the values match the ones calculated above.

The Precision/Recall Trade-off

As we saw above, by default, sklearn finds a model with approximately equal precision and recall.

But suppose you want asymmetric results--consider a facial recognition scanner to detect known thieves in a retail store. You really want to avoid False Positives, which accuse innocent shoppers of being thieves. It's worth accepting a higher False Negative rate to achieve that end.

Consider the situation shown below.

The neuron has an output signal level. Values above the threshold are classified as 5's, values below that level are classified as non-5's.

If the threshold is low, many outputs are classified as 5's, producing many False Positives. This makes Precision low.

If the threshold is high, many outputs are classified as non-5's, producing many False Negatives. This makes Recall low.

Viewing the Neuron Output

Execute these commands to see the neuron output signal from our model for the first twelve images: 
for i in range(12):
  print(y[i], sgd_clf.decision_function([X[i]])[0])
As shown below, the signal is only above zero for the first and last images, which are the 5's.

By default, sklearn uses a threshold of zero.

If the threshold were 3000, the first image would be classified as non-5, but the last one would still be classified as 5.

Using Different Threshold Values

Execute these commands to : 
y_scores = cross_val_predict(sgd_clf, X_train, y_train_5, cv=3,
                             method="decision_function")
from sklearn.metrics import precision_recall_curve
precisions, recalls, thresholds = precision_recall_curve(y_train_5, y_scores)

import matplotlib.pyplot as plt
plt.plot(thresholds, precisions[:-1], "b--", label="Precision", linewidth=2)
plt.plot(thresholds, recalls[:-1], "g-", label="Recall", linewidth=2)
plt.grid()
plt.axis([-50000, 50000, 0, 1])
plt.xlabel("Threshold")
plt.legend(loc="center right")
plt.show()
As shown below, as the threshold increases, Precision increases and Recall decreases.

Precision gets noisy at high thresholds, where there are very few positives.

Flag ML 105.1: Precision for Threshold 3000 (15 pts)

Execute these commands to calculate the Precision and Recall for a threshold of 0: 
y_train_pred = (y_scores >= 0)
print("Precision:", precision_score(y_train_5, y_train_pred))
print("Recall:", recall_score(y_train_5, y_train_pred))
print("F1 Score:", f1_score(y_train_5, y_train_pred))
As shown below, you get the same values we saw above, because the default threshold is zero.
Change the zero in the first line to 3000 and calculate the values again. The flag is the precision value, covered by a green rectangle in the image below.

Multiclass Classification with SVC

Execute these commands to create and train a C-Support Vector Classification (SVC) model to classify the images into all ten categores, not just "5" and "non-5". It will automatically choose the One-versus-One (OvO) strategy, with 45 binary classifiers. 
from sklearn.svm import SVC

svm_clf = SVC(random_state=42)
svm_clf.fit(X_train[:2000], y_train[:2000])  # y_train, not y_train_5

count_false = 0
for i in range(100):
  scores = svm_clf.decision_function([X[i]])[0]
  correct = y_train[i] == svm_clf.predict([X[i]])[0]
  if correct == False:
    print(i, correct, y_train[i], scores.round(2))
    count_false += 1
print("Count of false:", count_false)
The first column is the right answer, and the second column is "True" if the classification was correct and "False" of it was not. Only the "False" rows are printed.

As shown below, it was wrong only 3 times, and one of those errors was image 80. Image 80 was actually a "9", but was classified as a "0" because the decision function for "0" was 9.27, the largest value on that line.

Using One-versus-the-Rest (OvR)

Execute these commands to create and train a model forcing it to use the OvR strategy: 
from sklearn.multiclass import OneVsRestClassifier

ovr_clf = OneVsRestClassifier(SVC(random_state=42))
ovr_clf.fit(X_train[:2000], y_train[:2000])

count_false = 0
for i in range(100):
  scores = ovr_clf.decision_function([X[i]])[0]
  correct = y_train[i] == ovr_clf.predict([X[i]])[0]
  if correct == False:
    print(i, correct, y_train[i], scores.round(2))
    count_false += 1
print("Count of false:", count_false)
As shown below, the model now gets only 2 of the first 100 training images wrong.

Multiclass Classification with SGD

Execute these commands to create and train a Stochastic Gradient Descent (SGD) model to classify the images: 
from sklearn.linear_model import SGDClassifier

sgd_clf = SGDClassifier(random_state=42)
sgd_clf.fit(X_train[:2000], y_train[:2000]) 

count_false = 0
for i in range(500):
  scores = sgd_clf.decision_function([X[i]])[0] / 1000.0
  correct = y_train[i] == sgd_clf.predict([X[i]])[0]
  if correct == False:
    print(i, correct, y_train[i], scores.round())
    count_false += 1
print("Count of false:", count_false)
The decision functions have been divided by 1000 to make the display easier to read.

As shown below, this model is much better, making only 5 errors in the first 500 training images!

Overfitting

Is this model really so much better?

Execute these commands to examine its performance on the test set: 

count_false = 0
for i in range(500):
  scores = sgd_clf.decision_function([X_test[i]])[0] / 1000.0
  correct = y_test[i] == sgd_clf.predict([X_test[i]])[0]
  if correct == False:
    count_false += 1
print("Count of false:", count_false)
As shown below, it's much worse at the test set, making 91 errors in the first 500 test images. This is a case of overfitting--matching the training data too closely, but not the test data.

Cross-Validation

Execute these commands to evaluate the model using cross-validation with three folds: 
from sklearn.model_selection import cross_val_score

cross_val_score(sgd_clf, X_train[:2000], y_train[:2000], cv=3, scoring="accuracy")
As shown below, the accuracy is over 80% on each fold.

Scaling the Inputs

Execute these commands to scale the inputs and recalculate the cross-validation: 
from sklearn.preprocessing import StandardScaler

scaler = StandardScaler()
X_train_scaled = scaler.fit_transform(X_train[:2000].astype("float64"))
cross_val_score(sgd_clf, X_train_scaled, y_train[:2000], cv=3, scoring="accuracy")
As shown below, the accuracy improves by only about 1% on each fold.

Confusion Matrix

Execute these commands to examine its performance on the test set: 
from sklearn.metrics import ConfusionMatrixDisplay

y_train_pred = cross_val_predict(sgd_clf, X_train_scaled, y_train[:2000], cv=3)
ConfusionMatrixDisplay.from_predictions(y_train[:2000], y_train_pred)
plt.show()
As shown below, most images are on the main diagonal, indicating correct predictions.

Confusion Matrix Normalized by Row

The matrix above shows raw counts of images. To see it in percentages, execute these commands: 
ConfusionMatrixDisplay.from_predictions(y_train[:2000], y_train_pred,
                                        normalize="true", values_format=".0%")
plt.show()
As shown below, the most common error was misclassifying a "9" as a "7", and this happened 7% of the time.

Flag ML 105.2: Correct Percentage for 0's (10 pts)

The flag is covered by a red rectangle in the image below.

Sources

Hands-On Machine Learning with Scikit-Learn, Keras, and TensorFlow 3rd Edition, Kindle Edition

Posted and video added 4-20-23
parser="auto" added to fetch_openml() call 4-26-23
Minor typo fixed 5-3-23
Video updated 5-3-23
Typo fixed 9-8-23
Another minor text error fixed 7-23-24