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The Naive Bayes classifier is a probabilistic classifier that is based on the Bayes theorem. The classifier is generally preferred for high-dimensional data sets due to its simplicity and speed.

In this lesson, we will focus on an intuitive explanation of how Naive Bayes classifiers work, followed by its implementation in Python using a real-world dataset.

**Bayes Theorem**

The Bayes theorem describes the probability of occurrence of an event, based on prior knowledge of conditions that might be related to the event. It is stated mathematically as the following equation,

[latexpage]$$P(A | B) = \frac{P(B | A) P(A)}{P(B)}$$

where,

– $A$ and $B$ are events and $P(B)!=0$,

– $P(A | B)$ is a conditional probability that defines the likelihood of event $A$ occurring given that $B$ is true, also – known as *posterior probability,*

– $P(B | A)$ is a conditional probability that defines the likelihood of event $B$ occurring given that $A$ is true, also known as the *likelihood*, and

– $P(A)$ and $P(B)$ are the probabilities of observing $A$ and $B$ independently of each other, also known as *prior probability*

**How does Naive Bayes Classifier work? **

For output label $y$ as $L$ and features $x_1, x_2, \dots, \x_n$ as $F$, the Bayes’s Theorem can be written as,

$$P(y | (x_1, x_2, \dots, x_n)) = P(L | F) = \frac{P(F | L ) * P( L )}{P(F)}$$

where

– $P(L | F)$ is the probability of an output class ($y$) given the provided data or features $(x_1, x_2, \dots, x_n)$, known as the *posterior probability*,

– $P(F | L)$ is the probability of features $(x_1, x_2, \dots, x_n)$ given an output class ($y$), known as the *likelihood*, and

– $P(F)$ and $P(L)$ are the probabilities of observing $F$ and $L$ independently of each other, also known as the *prior probability*

The Naive Bayes Classifier assumes that a particular feature in a class is independent of other features due to which it gets its name to be “Naive”. The presence or absence of a feature does not affect other features in the data. It calculates the probability of an event in the following simple steps:

**Step 1:** Calculate the prior probability for given class labels, i.e., the class prior probability ($P(L)$), and the predictor prior probability ($P(F)$).

**Step 2:** Find likelihood probability with each attribute for each class, i.e., $P(F | L)$ for each set of features in $F$.

**Step 3:** Use Bayes Theorem to find the posterior probability, i.e., $P(L | F)$.

**Step 4:** The class with a higher probability, given the input, belongs to the higher probability class.

**Naive Bayes Classifier in Python**

Now that we know the basic idea of Naive Bayes Classifier, we will now discuss a step-wise Python implementation of the algorithm.

**1. Importing necessary libraries**

Before we begin to build our model, let us import some essential Python libraries for mathematical calculations, data loading, preprocessing, and model development and prediction.

# Importing the libraries import numpy as np import pandas as pd import matplotlib.pyplot as plt %matplotlib inline # scikit-learn modules from sklearn.datasets import load_breast_cancer from sklearn.model_selection import train_test_split from sklearn.preprocessing import StandardScaler from sklearn.naive_bayes import GaussianNB from sklearn.metrics import confusion_matrix, accuracy_score, classification_report # For plotting the classification results from mlxtend.plotting import plot_decision_regions

**2. Importing the dataset**

For this problem, we will be loading the Breast Cancer dataset from scikit-learn. The dataset consists of data related to breast cancer patients and their diagnosis (malignant or benign).

# Importing the dataset dataset = load_breast_cancer() # Converting to pandas DataFrame df = pd.DataFrame(dataset.data, columns = dataset.feature_names) df['target'] = pd.Series(dataset.target) df.head()

print("Total samples in our dataset is: {}".format(df.shape[0]))

Total samples in our dataset is: 569

dataset.describe()

**3. Separating the features and target variable**

After loading the data set, the independent variable ($x$) and the dependent variable ($y$) need to be separated. Our concern is to find the relationships between the features and the target variable from the above dataset.

For this implementation example, we will only be using the ‘mean perimeter’ and ‘mean texture’ features but you can certainly use all of them.

# Selecting the features features = ['mean perimeter', 'mean texture'] x = df[features] # Target Variable y = df['target']

**4. Splitting the dataset into training and test set **

After separating the independent variables ($x$) and dependent variable $(y)$, these values are split into train and test sets to train and evaluate the linear model. We use the train_test_split() module of scikit-learn for splitting the available data into an 80-20 split. We will be using twenty percent of the available data as the test set and the remaining data as the train set.

# Splitting the dataset into the training and test set x_train, x_test, y_train, y_test = train_test_split(x, y, test_size = 0.20, random_state = 25 )

**5. Fitting the model to the training set**

For building the model, we will be using the Gaussian Naive Bayes model. It is the easiest Naive Bayes classifier that assumes that the data from each label is drawn from a simple Gaussian distribution. After splitting the data into dependent and independent variables, the Naive Bayes model is fitted with the training data using the GaussianNB() class from scikit-learn.

# Fitting Naive Bayes to the Training set model = GaussianNB() model.fit(x_train, y_train)

GaussianNB()

**6. Predicting the test results**

Finally, the model is tested on the data to get the predictions.

# Predicting the results y_pred = model.predict(x_test)

**7. Evaluating the model**

Let us now evaluate the model using confusion matrix and calculate its classification accuracy. Confusion matrix determines the performance of the predicted model. Other metrics such as the precision, recall and f1-score are given by the classification report module of scikit-learn.

**Precision** defines the ratio of correctly predicted positive observations of the total predicted positive observations. It defines how accurate the model is. **Recall** defines the ratio of correctly predicted positive observations to all observations in the actual class. **F1 Score** is the weighted average of Precision and Recall and is often used as a metric in place of accuracy for imbalanced datasets.

# Confusion matrix print("Confusion Matrix") matrix = confusion_matrix(y_test, y_pred) print(matrix) # Classification Report print("\nClassification Report") report = classification_report(y_test, y_pred) print(report) # Accuracy of the model accuracy = accuracy_score(y_test, y_pred) print('Gaussian NB Classification Accuracy of the model: {:.2f}%'.format(accuracy*100))

Confusion Matrix [[30 9] [ 5 70]] Classification Report precision recall f1-score support 0 0.86 0.77 0.81 39 1 0.89 0.93 0.91 75 accuracy 0.88 114 macro avg 0.87 0.85 0.86 114 weighted avg 0.88 0.88 0.88 114 Gaussian NB Classification Accuracy of the model: 87.72%

Hence, the model is working quite well with an accuracy of *87.72%*.

**8. Plotting the decision boundary**

We will now plot the decision boundary of the model on test data.

# Plotting the decision boundary plot_decision_regions(x_test.values, y_test.values, clf = model, legend = 2) plt.title("Decision boundary using Naive Bayes (Test)") plt.xlabel("mean_perimeter") plt.ylabel("mean_texture")

Hence, the plot shows the distinction between the two classes as classified by the Naive Bayes Classification algorithm in Python.

**Putting it all together**

The final code for the implementation of **Naive Bayes Classification in Python** is as follows.

# Importing the libraries import numpy as np import pandas as pd import matplotlib.pyplot as plt %matplotlib inline # scikit-learn modules from sklearn.datasets import load_breast_cancer from sklearn.model_selection import train_test_split from sklearn.preprocessing import StandardScaler from sklearn.naive_bayes import GaussianNB from sklearn.metrics import confusion_matrix, accuracy_score, classification_report # Plotting the classification results from mlxtend.plotting import plot_decision_regions # Importing the dataset dataset = load_breast_cancer() # Converting to pandas DataFrame df = pd.DataFrame(dataset.data, columns = dataset.feature_names) df['target'] = pd.Series(dataset.target) print("Total samples in our dataset is: {}".format(df.shape[0])) # Describe the dataset df.describe() # Selecting the features features = ['mean perimeter', 'mean texture'] x = df[features] # Target variable y = df['target'] # Splitting the dataset into the training and test set x_train, x_test, y_train, y_test = train_test_split(x, y, test_size = 0.20, random_state = 25 ) # Fitting Naive Bayes to the Training set model = GaussianNB() model.fit(x_train, y_train) # Predicting the results y_pred = model.predict(x_test) # Confusion matrix print("Confusion Matrix") matrix = confusion_matrix(y_test, y_pred) print(matrix) # Classification Report print("\nClassification Report") report = classification_report(y_test, y_pred) print(report) # Accuracy of the model accuracy = accuracy_score(y_test, y_pred) print('Gaussian NB Classification Accuracy of the model: {:.2f}%'.format(accuracy*100)) # Plotting the decision boundary plt.figure(figsize=(10,6)) plot_decision_regions(x_test.values, y_test.values, clf = model, legend = 2) plt.title("Decision boundary using Naive Bayes (Test)") plt.xlabel("mean_perimeter") plt.ylabel("mean_texture")

In this lesson, we discussed Naive Bayes Classifier along with its implementation in Python.

We will now discuss some tree-based methods for classification such as the Decision Tree and Random Forest in the upcoming lessons.