The variance of X, denoted by Var (X) or
σx
2, is defined as 
2
2
1
Var(X)=
(
μ)
(
)
n
x
i
i
i
x
p x
 
 
 
 
 
or equivalently  σx
2 = E (X – μ)2
The non-negative number
2
1
Va r(X) =
(
μ)
(
)
n
x
i
i
i
x
p x
 
 
 
 
 
is called the standard deviation of the  random variable X � Var (X) = E (X2) – [E(X)]2
� Trials of a random experiment are called Bernoulli trials, if they satisfy the
following conditions :
(i) There should be a finite number of trials (ii) The trials should be independent (iii) Each trial has exactly two outcomes : success or failure