The Naive Bayes classifier is a frequently encountered term in the blog posts here; it has been used in the previous articles for building an email spam filter and for performing sentiment analysis on movie reviews. Thus a post explaining its working has been long overdue. Despite being a fairly simple classifier with oversimplified assumptions, it works quite well in numerous applications. Let us now try to unravel its working and understand what makes this family of classifiers so popular.

We begin by refreshing our understanding about the fundamental concepts behind the classifier – conditional probability and the Bayes’ theorem. This is followed by an elementary example to show the various calculations which are made to arrive at the classification output. Finally, we implement the classifier’s algorithm in Python and then validate the code’s output with results obtained for the demonstrated example.

## Conditional Probability and Bayes’ Theorem

In the simplest terms, conditional probability, denoted as $P(O|E)$ and read as probability of outcome O given event E, is the probability of the occurrence of an outcome O given that some other event E has already occurred. It is calculated as

$P(O|E) =$ $\frac{P(O\thinspace\cap E)}{P(E)}$

$P(O\cap E)$ is the probability that both O and E occur; this is the same as calculating $P(O)$, the probability of O occurring multiplied by $P(E|O)$, the probability of E occurring given that O has already occurred, or conversely $P(E)$, the probability of E occurring multiplied by $P(O|E)$, the probability of O occurring given that E has occurred.

$P(O\cap E) = P(O) * P(E|O) = P(E) * P(O|E)$

The equality of the two terms on the right leads to the Bayes’ Theorem.

$P(O|E) =$ $\frac{P(E|O) \thinspace*\thinspace P(O)}{P(E)}$

Thus, the Bayes’ Theorem is a way to go from $P(O|E)$ to $P(E|O)$. The above equation is often written as

$posterior \thinspace probability =$ $\frac{likelihood\thinspace*\thinspace prior\thinspace probability}{evidence}$

$P(O)$ is also known as a priori probability because it is probability of the class (or outcome or model) which is always known a priori from the training examples, while $P(O|E)$ is called the a posteriori probability because it is the probability of the class (or model) which we calculate after seeing the data (or evidence or events). Likelihood answers the question that “What is the probability of the data (or evidence or events) that it has been generated from the model (or class or outcome) ?”

### Why is it called “Naive”?

So far, we have only talked about the effect of a single event E on the occurrence of outcome O. In reality however, the probability of outcome O’s occurrence is governed by more than one already occurred event. The mathematics behind such a scenario gets very complicated and is made simple by assuming that the various governing events are independent of each other. Mathematically expressing, if an outcome O depends on events $E_1$, $E_2$,…, $E_n$, then assuming $E_1$, $E_2$,…, $E_n$ do not depend on one other,

$P(O|E_1, E_2,..., E_n)=$ $\frac{P(E_1|O) \thinspace * \thinspace P(E_2|O) \thinspace * \thinspace ... \thinspace * \thinspace P(E_n|O)} {P(E_1) \thinspace * \thinspace P(E_2) \thinspace * \thinspace... \thinspace *\thinspace P(E_n)}$ $P(O)$             (1)

Thus, the model is very naively believing that the effect of the occurrence of any of the events is completely independent of the occurrence of other events. This is obviously not the case with most of the real world problems and the predictor features are known to influence and be influenced by other features. Still, naive Bayes is known to provide results at par and sometimes even better than highly complex and computationally expensive classification models.

## An Illustrative Example

Lets clear our understanding by taking a simple example. Suppose we have the following training data (taken from this YouTube video) regarding whether a patient had the flu or not, depending on if he had chills, a runny nose, headache and fever .

You might have guessed that chills, runny nose, headache and fever are features (analogous to the already occurred events $E_1, E_2, E_3$ and $E_4$ explained above), while flu and no flu are the two classes $C_1$ and $C_2$ in which the above patients were known to be falling (analogous to the two outcomes $O_1$ and $O_2$). We get a new patient with the following symptoms which are previously unseen by the classifier :

How to decide whether the patient has flu or not using the naive Bayes’ classifier?

### How is the classification done?

For the task of classification, we use the training data to determine the prior probabilities of the 2 classes (flu or no flu) and the likelihoods of the 4 events that govern these outcomes. We begin by calculating the likelihoods of each of the event given the class, like $P(chills = N| flu = Y)$, $P(fever = N| flu = N)$ and so on, just like illustrated in equation 1 above. These values are to be used to determine the posterior probability of the test data belonging to each of the 2 classes, i.e., $P(C_1|E_1, E_2, E_3, E_4)$ and $P(C_2|E_1, E_2, E_3, E_4)$. The patient is predicted to be belonging to the class for which the value of the calculated posterior probability is higher.

The various probabilities that we are going to require as per equation 1 for the case when the patient has flu, ie, $flu = Y$ are calculated as below..

 $P(flu = Y)$ $\frac{5}{8} = 0.625$ ($P(O)$) $P(chills = Y|flu = Y)$ $\frac{3}{5} = 0.6$ ($P(E_1|O)$) $P(runnynose = N|flu = Y)$ $\frac{1}{5} = 0.2$ ($P(E_2|O)$) $P(headache = mild|flu = Y)$ $\frac{2}{5} = 0.4$ ($P(E_3|O)$) $P(fever = N|flu = Y)$ $\frac{1}{5} = 0.2$ ($P(E_1|O)$) $P(chills = Y)$ $\frac{4}{8} = 0.5$ ($P(E_1)$) $P(runnynose = N)$ $\frac{3}{8} = 0.375$ ($P(E_2)$) $P(headache = mild)$ $\frac{3}{8} = 0.375$ ($P(E_3)$) $P(fever = N)$ $\frac{3}{8} = 0.375$ ($P(E_4)$)

Now calculating $P(flu = Y|E_1, E_2, E_3, E_4)$ by plugging these probabilities in (1)

$= \frac{0.6 \thinspace *\thinspace 0.2 *\thinspace 0.4 \thinspace* \thinspace0.2}{0.5 \thinspace*\thinspace 0.375 \thinspace*\thinspace 0.375 \thinspace*\thinspace 0.375}$ $0.625=\thinspace 0.227$

Similarly, the various probabilities are calculated for the case of $flu = N$. The readers are encouraged to calculate these values and check them with the ones used below. Hence $P(flu = N|E_1, E_2, E_3, E_4)$

$= \frac{0.33 \thinspace *\thinspace 0.66 *\thinspace 0.33 \thinspace* \thinspace0.66}{0.5 \thinspace*\thinspace 0.375 \thinspace*\thinspace 0.375 \thinspace*\thinspace 0.375}$ $0.375=\thinspace 0.674$

Thus the naive Bayes’ classifier will predict the class $flu = N$ for the above patient because the value of posterior probability was higher for $flu = N$ than for the class $flu = Y$.

If you notice, you can see what the task of prediction eventually reduces to – just calculating some probabilities and a few multiplications. All fairly easy and quick. If you pay even more attention, you see that since the denominator is the same while determining the probabilities of the outcome as either flu or no flu, you can even do away without calculating its value.

Thus $P(flu = Y|E_1, E_2, E_3, E_4)$$0.6 * 0.4 * 0.2 * 0.2 * 0.625 = \textbf{0.006}$

$P(flu = N|E_1, E_2, E_3, E_4)$ = $0.33 * 0.66 * 0.33 * 0.66 * 0.375 = \textbf{0.0185}$.

Again, the calculated probabilities give the same prediction as before, ie, no flu. We will validate these 2 values in the next section via a Python code.

This is what makes naive Bayes’ so popular as a classifier, combined with the fact that it has been seen to perform exceptionally well in many applications.

## Python implementation

Lets cross check our values by writing a Python script for the above example.

Foremost, we build the training data from Figure 1. For the features, we provide label 1 to all Y (ie, the symptom was present in the patient) and 0 to all N (ie, the symptom was not present in the patient). Also, in the case of the feature “headache”, no headache = 0, mild = 1 and strong = 2. The outcome also follows the same pattern and thus, no flu = 0 and flu = 1.

import numpy as np
training = np.asarray(((1,0,1,1),
(1,1,0,0),
(1,0,2,1),
(0,1,1,1),
(0,0,0,0),
(0,1,2,1),
(0,1,2,0),
(1,1,1,1)));

outcome = np.asarray((0,1,1,1,0,1,0,1))


We begin by calculating the prior probabilities of the two classes. We use a dictionary for the same.

from collections import Counter, defaultdict
def occurrences(outcome):
no_of_examples = len(outcome)
prob = dict(Counter(outcome))
for key in prob.keys():
prob[key] = prob[key] / float(no_of_examples)
return prob


When we call occurrences(classes), we get the following prior probabilities for the 2 classes.

Prior probabilities = {0: 0.375, 1: 0.625}

Now we need to count the occurrences of the features class-wise. For this, we again employ a dictionary and separate the vectors of the 2 classes, by looking at a subset of the training set, taking one class at a time.

classes     = np.unique(outcome)
rows, cols  = np.shape(training)
likelihoods = {}
for cls in classes:
#initializing the dictionary
likelihoods[cls] = defaultdict(list)

for cls in classes:
#taking samples of only 1 class at a time
row_indices = np.where(outcome == cls)[0]
subset = training[row_indices, :]
r, c = np.shape(subset)
for j in range(0, c):
likelihoods[cls][j] += list(subset[:,j])


If you look at likelihoods when the code has run, it will look as follows:

{0: defaultdict(, {0: [1, 0, 0], 1: [0, 0, 1], 2: [1, 0, 2], 3: [1, 0, 0]}),
1: defaultdict(, {0: [1, 1, 0, 0, 1], 1: [1, 0, 1, 1, 1], 2: [0, 2, 1, 2, 1], 3: [0, 1, 1, 1, 1]})}

For both the classes 0 and 1, and for each of the 4 features, we have thus saved the occurrences in likelihoods. For example, for the class 0 (no flu), and feature 1 (ie, runny nose), the occurrences are 0, 0 and 1. Now what remains is to calculate the probabilities, per class, per feature.

for cls in classes:
for j in range(0,c):
likelihoods[cls][j] = occurrences(likelihoods[cls][j])


likelihoods will now contain the probabilities per feature per class.

The class of a previously unseen example will be determined by the following code.

results = {}
for cls in classes:
class_probability = class_probabilities[cls]
for i in range(0,len(new_sample)):
relative_feature_values = likelihoods[cls][i]
if new_sample[i] in relative_feature_values.keys():
class_probability *= relative_feature_values[new_sample[i]]
else:
class_probability *= 0
results[cls] = class_probability


For the test case in Figure 2, ie,  new_sample = [1 0 1 0], we get

results = {0: 0.018518518518518517, 1: 0.006000000000000002}

, which are the same as we had calculated in theory.

The entire code is as follows:

import numpy as np
from collections import Counter, defaultdict

def occurrences(list1):
no_of_examples = len(list1)
prob = dict(Counter(list1))
for key in prob.keys():
prob[key] = prob[key] / float(no_of_examples)
return prob

def naive_bayes(training, outcome, new_sample):
classes     = np.unique(outcome)
rows, cols  = np.shape(training)
likelihoods = {}
for cls in classes:
likelihoods[cls] = defaultdict(list)

class_probabilities = occurrences(outcome)

for cls in classes:
row_indices = np.where(outcome == cls)[0]
subset      = training[row_indices, :]
r, c        = np.shape(subset)
for j in range(0,c):
likelihoods[cls][j] += list(subset[:,j])

for cls in classes:
for j in range(0,cols):
likelihoods[cls][j] = occurrences(likelihoods[cls][j])

results = {}
for cls in classes:
class_probability = class_probabilities[cls]
for i in range(0,len(new_sample)):
relative_values = likelihoods[cls][i]
if new_sample[i] in relative_values.keys():
class_probability *= relative_values[new_sample[i]]
else:
class_probability *= 0
results[cls] = class_probability
print results

if __name__ == "__main__":
training   = np.asarray(((1,0,1,1),(1,1,0,0),(1,0,2,1),(0,1,1,1),(0,0,0,0),(0,1,2,1),(0,1,2,0),(1,1,1,1)));
outcome    = np.asarray((0,1,1,1,0,1,0,1))
new_sample = np.asarray((1,0,1,0))
naive_bayes(training, outcome, new_sample)


## Final Thoughts

Hope the bog-post was easy to follow and gives you a good understanding of Naive Bayes classifiers.
The naive Bayes (NB) classifier is a probabilistic classifier that assumes complete independence between the predictor features. They perform quite well even though the working behind them is simplistic. There exist other methods as well to determine the likelihoods of the features. Thus the NB classifiers are actually a family of classifiers rather being just a single classifier. Below are the classifiers available in any machine learning library, such as sklearn for Python.

1. Gaussian NB – The likelihood of the features is assumed to be Gaussian distribution.
2. Multinomial NB – This works for multinomially distributed data like word count vector where features in the vectors are probability of the event appearing in the sample.
3. Bernoulli NB – This is used for multivariate Bernoulli distributions; i.e., there may be multiple features but each one is assumed to be a binary-valued (boolean) variable.

I will strongly recommend learners to read about the likelihood calculations of above variants in order to understand more practical usage of Naive Bayes classifier in various applications like sentiment analysis, email spam filters, etc.

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Happy machine learning