7.5.3.2 Probability Notation for
Redundancy Computations
Reliability of redundancy combinations is expressed in
probabilistic terms of success or failure  for a given mission period, a
given number of operating cycles, or a given number of time independent
"events," as appropriate. The "MTBF" measure of reliability is not readily
usable because of the nonexponentiality of the reliability function produced
by redundancy. Reliability of redundancy combinations which are "time
dependent" is therefore computed at a discrete point in time, as a probability
of success for this discrete time period. The following notation is applicable
to all cases and is used throughout this section:
R 
= 
probability of
success or reliability of a unit or block

Q 
= 
= probability of failure or
unreliability of a unit or block

p 
= 
probability of
success or reliability of an element

q 
= 
probability of
failure or unreliability of an
element 
For probability statements concerning an event:
P(A) = probability that A occurs
P( ) = probability that A does
not occur
For the above probabilities:
R + Q = 1
p + q = 1
P(A) + P( ) = 1