Noticing that the contribution variable 
has the same possible values as
e, we define functions 
by
for any value 
of e.
We shall refer to 
as the
contributing factor of 
to e.
By using the operator 
, we can now rewrite equation
(2) as follows
It is interesting to notice the similarity between equation (1) and equation (5). In equation (1) conditional independence allows one to factorize a joint probability into factors that involve less variables, while in equation (5) causal independence allows one to factorize a conditional probability into factors that involve less variables. However, the ways by which the factors are combined are different in the two equations.
