Naive Bayes: A Baseline Model for Machine Learning Classification Performance
We can use Pandas to conduct Bayes Theorem and Scikitlearn to implement the Naive Bayes Algorithm. We take a step by step approach to understand Bayes and implementing the different options in Scikitlearn.
Bayes Theorem
The above equation represents Bayes Theorem in which it describes the probability of an event occurring P(A) based on our prior knowledge of events that may be related to that event P(B).
Lets explore the parts of Bayes Theorem:
 P(AB)  Posterior Probability
 The conditional probability that event A occurs given that event B has occurred.
 P(A)  Prior Probability
 The probability of event A.
 P(B)  Evidence
 The probability of event B.
 P(BA)  Likelihood
 The conditional probability of B occurring given event A has occurred.
Now, lets explore the parts of Bayes Theorem through the eyes of someone conducting machine learning:
 P(AB)  Posterior Probability
 The conditional probability of the response variable (target variable) given the training data inputs.
 P(A)  Prior Probability
 The probability of the response variable (target variable).
 P(B)  Evidence
 The probability of the training data.
 P(BA)  Likelihood
 The conditional probability of the training data given the response variable.
 P(cx)  Posterior probability of the target/class (c) given predictors (x).
 P(c)  Prior probability of the class (target).
 P(xc)  Probability of the predictor (x) given the class/target (c).
 P(x)  Prior probability of the predictor (x).
Example of using Bayes theorem:
I'll be using the tennis weather dataset.
import numpy as np import pandas as pd import matplotlib.pyplot as plt %matplotlib inline
tennis = pd.read_csv('tennis.csv') tennis
outlook  temp  humidity  windy  play  

0  sunny  hot  high  False  no 
1  sunny  hot  high  True  no 
2  overcast  hot  high  False  yes 
3  rainy  mild  high  False  yes 
4  rainy  cool  normal  False  yes 
5  rainy  cool  normal  True  no 
6  overcast  cool  normal  True  yes 
7  sunny  mild  high  False  no 
8  sunny  cool  normal  False  yes 
9  rainy  mild  normal  False  yes 
10  sunny  mild  normal  True  yes 
11  overcast  mild  high  True  yes 
12  overcast  hot  normal  False  yes 
13  rainy  mild  high  True  no 
Lets take a look at how each category looks when inside a frequency table:
outlook = tennis.groupby(['outlook', 'play']).size() temp = tennis.groupby(['temp', 'play']).size() humidity = tennis.groupby(['humidity', 'play']).size() windy = tennis.groupby(['windy', 'play']).size() play = tennis.play.value_counts()
print(temp) print('') print(humidity) print('') print(windy) print('') print(outlook) print('') print('play') print(play)
temp play
cool no 1
yes 3
hot no 2
yes 2
mild no 2
yes 4
dtype: int64

humidity play
high no 4
yes 3
normal no 1
yes 6
dtype: int64

windy play
False no 2
yes 6
True no 3
yes 3
dtype: int64

outlook play
overcast yes 4
rainy no 2
yes 3
sunny no 3
yes 2
dtype: int64

play
yes 9
no 5
Name: play, dtype: int64
What is the probability of playing tennis given it is rainy?
 P(rainplay=yes)
 frequency of (outlook=rainy) when (play=yes) / frequency of (play=yes) = 3/9
 P(play=yes)
 frequency of (play=yes) / total(play) = 9/14
 P(outlook=rainy)
 frequency of (outlook=rainy) / total(outlook) = 5/14
(3/9)*(9/14)/(5/14)
0.6
The probability of playing tennis when it is rainy is 60%. The process is very simple once you obtain the frequencies for each category.
Here is a simple function to help any newbies remember the parts of Bayes equation:
def bayestheorem(): print('Posterior [P(cx)]  Posterior probability of the target/class (c) given predictors (x)'), print('Prior [P(c)]  Prior probability of the class (target)'), print('Likelihood [P(xc)]  Probability of the predictor (x) given the class/target (c)'), print('Evidence [P(x)]  Prior probability of the predictor (x))')
Here is a simple function to calculate the posterior probability for you, but you must be able to find each part of bayes equation yourself.
def bayesposterior(prior, likelihood, evidence, string): print('Prior=', prior), print('Likelihood=', likelihood), print('Evidence=', evidence), print('Equation =','(Prior*Likelihood)/Evidence') print(string, (prior*likelihood)/evidence)
Lets see another way to find the posterior probability this time using contingency tables in Python:
ct = pd.crosstab(tennis['outlook'], tennis['play'], margins = True) print(ct)
no yes rowtotal
overcast 0 4 4
rainy 2 3 5
sunny 3 2 5
coltotal 5 9 14
ct.columns = ["no","yes","rowtotal"] ct.index= ["overcast","rainy","sunny","coltotal"] ct / ct.loc["coltotal","rowtotal"]
no  yes  rowtotal  

overcast  0.000000  0.285714  0.285714 
rainy  0.142857  0.214286  0.357143 
sunny  0.214286  0.142857  0.357143 
coltotal  0.357143  0.642857  1.000000 
To only get the column total
ct / ct.loc["coltotal"]
no  yes  rowtotal  

overcast  0.0  0.444444  0.285714 
rainy  0.4  0.333333  0.357143 
sunny  0.6  0.222222  0.357143 
coltotal  1.0  1.000000  1.000000 
To only get the row total
ct.div(ct["rowtotal"], axis=0)
no  yes  rowtotal  

overcast  0.000000  1.000000  1.0 
rainy  0.400000  0.600000  1.0 
sunny  0.600000  0.400000  1.0 
coltotal  0.357143  0.642857  1.0 
These tables are all pandas dataframe objects. Therefore using pandas subsetting and the bayesposterior
function I made, we can arrive at the same conclusion:
bayesposterior(prior = ct.iloc[1,1]/ct.iloc[3,1], likelihood = ct.iloc[3,1]/ct.iloc[3,2], evidence = ct.iloc[1,2]/ct.iloc[3,2], string = 'Probability of Tennis given Rain =')
Prior= 0.3333333333333333
Likelihood= 0.6428571428571429
Evidence= 0.35714285714285715
Equation = (Prior*Likelihood)/Evidence
Probability of Tennis given Rain = 0.6
Naive Bayes Algorithm
Naive Bayes is a supervised Machine Learning algorithm inspired by the Bayes theorem. It works on the principles of conditional probability. Naive Bayes is a classification algorithm for binary and multiclass classification. The Naive Bayes algorithm uses the probabilities of each attribute belonging to each class to make a prediction.
Example
What is the probability of playing tennis when it is sunny, hot, highly humid and windy? So using the tennis dataset, we need to use the Naive Bayes method to predict the probability of someone playing tennis given the mentioned weather conditions.
pd.crosstab(tennis['outlook'], tennis['play'], margins = True)
play  no  yes  All 

outlook  
overcast  0  4  4 
rainy  2  3  5 
sunny  3  2  5 
All  5  9  14 
pd.crosstab(tennis['temp'], tennis['play'], margins = True)
play  no  yes  All 

temp  
cool  1  3  4 
hot  2  2  4 
mild  2  4  6 
All  5  9  14 
pd.crosstab(tennis['humidity'], tennis['play'], margins = True)
play  no  yes  All 

humidity  
high  4  3  7 
normal  1  6  7 
All  5  9  14 
pd.crosstab(tennis['windy'], tennis['play'], margins = True)
play  no  yes  All 

windy  
False  2  6  8 
True  3  3  6 
All  5  9  14 
pd.crosstab(index=tennis['play'],columns="count", margins=True)
col_0  count  All 

play  
no  5  5 
yes  9  9 
All  14  14 
Now by using the above contingency tables, we will go through how the Naive Bayes algorithm calculates the posterior probability.
 Calculate P(xplay=yes). In this case x refers to all the predictors 'outlook', 'temp', 'humidity' and 'windy'.
 P(sunnyplay=yes)→2/9
 P(hotplay=yes)→2/9
 P(highplay=yes)→3/9
 P(Trueplay=yes)→3/9
 Calculate P(xplay=no) using the same method as above.
 P(sunnyplay=no)→3/5
 P(hotplay=no)→2/5
 P(highplay=no)→4/5
 P(Trueplay=no)→3/5
 Calculate P(play=yes) and P(play=no)
 P(play=yes)→9/14
 P(play=yes)→5/14
 Calculate the probability of playing and not playing tennis given the predictors
 The prediction will be whichever probability is higher
p_x_yes = ((2/9)*(2/9)*(3/9)*(3/9)) print('The probability of the predictors given playing tennis is', '%.3f'%p_x_yes)
The probability of the predictors given playing tennis is 0.005
p_x_no = ((3/5)*(2/5)*(4/5)*(3/5)) print('The probability of the predictors given not playing tennis is ', '%.3f'%p_x_no)
The probability of the predictors given not playing tennis is 0.115
yes = (9/14) no = (5/14) print('The probability of playing tennis is', '%.3f'% yes) print('The probability of not playing tennis is', '%.3f'% no)
The probability of playing tennis is 0.643
The probability of not playing tennis is 0.357
yes_x = p_x_yes*yes print('The probability of playing tennis given the predictors is', '%.3f'%yes_x) no_x = p_x_no*no print('The probability of not playing tennis given the predictors is', '%.3f'%no_x)
The probability of playing tennis given the predictors is 0.004
The probability of not playing tennis given the predictors is 0.041
if yes_x > no_x: print('The probability of playing tennis when the outlook is sunny, the temperature is hot, there is high humidity and windy is higher') else: print('The probability of not playing tennis when the outlook is sunny, the temperature is hot, there is high humidity and windy is higher')
The probability of not playing tennis is higher when the outlook is sunny, the temperature is hot, there is high humidity and it is windy.
Type of Naive Bayes Algorithm
Python's Scikitlearn gives the user access to the following 3 Naive Bayes models.
 Gaussian
 The gaussian NB Alogorithm assumes all contnuous features (predictors) and all follow a Gaussian (Normal Distribution).
 Multinomial
 Multinomial NB is suited for discrete data that have frequencies and counts. Spam Filtering and Text/Document Classification are two very wellknown use cases.
 Bernoulli
 Bernoulli is similar to Multinomial except it is for boolean/binary features. Like the multinomial method it can be used for spam filtering and document classification in which binary terms (i.e. word occurrence in a document represented with True or False).
Lets implement a Multinomial and Gaussian Model with Scikitlearn
from sklearn.naive_bayes import GaussianNB, BernoulliNB, MultinomialNB from sklearn.model_selection import train_test_split from sklearn.metrics import *
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