6.4.4.2.1 Conventional
Probability Modeling Method
The conventional probability method may be used to prepare a
reliability mathematical model from a reliability block diagram. The
conventional probability method makes use of the equations developed for
redundancy to handle series, parallel, and seriesparallel combinations of
equipments. For nonseries parallel or complex configurations, use or repeated
use of the following equation is required.

P_{S} =
P_{S} (if X is good) R_{X} + P_{S} (if X
is bad) Q_{X} 
(6.19) 
where:
P_{S} = reliability of mission
P_{S} (if X
is good) = reliability of mission if X is good
P_{S} (if X is
bad) = reliability of mission if X is bad
R_{X} = reliability of
X
Q_{X} = unreliability of X = 1  R_{X}
In other words, the reliability of the mission is equal to the
reliability of the mission given a specific portion of the system works times
the probability that the portion of the system will work plus the reliability
of the mission given that the specific portion of the system fails times the
probability that the portion fails.
The above formula can also be used to generate probability of
success equations for seriesparallel configurations.