Bigram probability estimate of a word sequence, Probability estimation for a sentence using Bigram language model
Bigram Model  Probability Calculation  Example Problem
Solved Example:
Let us solve a small example to better understand
the Bigram model. For this we need a corpus and the test data. Let us assume
that the following is a small corpus;
Training
corpus:
<s> I am from
Vellore </s>
<s> I am a teacher
</s>
<s> students are
good and are from various cities</s>
<s> students from Vellore
do engineering</s>
Test
data:
<s> students are
from Vellore </s>
Let us find the Bigram probability of the
given test sentence. I explained the solution in two methods, just for the sake of understanding. the second method is the formal way of calculating the bigram probability of a sequence of words.
Method 1
As per the Bigram model, the test sentence can be expanded as follows to estimate the bigram probability;
Method 1
As per the Bigram model, the test sentence can be expanded as follows to estimate the bigram probability;
P(<s> students are
from Vellore </s>)
=
P(students  <s>) * P(are 
students) * P(from  are)
* P(Vellore  from) * P(</s>  Vellore)
* P(Vellore  from) * P(</s>  Vellore)
To
estimate bigram probabilities, we can use the following equation;
[Hint
– count of sentence start (<s>)
= 4, count of string <s> students = 1]
[Hint
– count of word students = 2, count
of string students are = 1]
[Hint – count of word are = 2, count of string are from = 1]
[Hint – count of word from = 3, count of string from Vellore = 2]
[Hint – count of word Vellore = 2, count of string Vellore </s> = 1]
P(<s>
students are from Vellore </s>)
= P(students
 <s>) * P(are  students) * P(from  are)
* P(Vellore  from) * P(</s>  Vellore)
* P(Vellore  from) * P(</s>  Vellore)
=
1/4 * 1/2 * 1/2 * 2/3 * 1/2 = 0.0208
Method 2
Formal
way of estimating the bigram probability of a word sequence:
The bigram probabilities of the test sentence
can be calculated by constructing Unigram and bigram probability count matrices
and bigram probability matrix as follows;
Unigram
count matrix
<s>

students

are

from

Vellore

4

2

2

3

2

Bigram
count matrix
w_{n}


students

are

from

Vellore

</s>


w_{n1}

<s>

1

0

0

0

0

students

0

1

1

0

0


are

0

0

1

0

0


from

0

0

0

2

0


Vellore

0

0

0

0

1

Bigram
probability matrix (normalized by unigram counts)
w_{n}


students

are

from

Vellore

</s>


w_{n1}

<s>

1/4

0/4

0/4

0/4

0/4

students

0/2

1/2

1/2

0/2

0/2


are

0/2

0/2

1/2

0/2

0/2


from

0/3

0/3

0/3

2/3

0/3


Vellore

0/2

0/2

0/2

0/2

1/2

P(<s>
students are from Vellore </s>)
= P(students
 <s>) * P(are  students) * P(from  are)
* P(Vellore  from) * P(</s>  Vellore)
* P(Vellore  from) * P(</s>  Vellore)
=
1/4 * 1/2 * 1/2 * 2/3 * 1/2 = 0.0208
The probability of the test sentence as per the bigram model is 0.0208.

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