The operational criticality factors method allocates failure
rates based on the system impact of a software
failure. Criticality is a measure of the system’s ability to continue to
operate and the system’s ability to be fail-safe. For certain modes of
operation, the criticality of that mode may call for a lower failure rate to
be allocated. In order to meet very low failure rates, fault-tolerance or
other methods may be needed.

(1) Determine the failure rate
goal of the software aggregate; l_{s}

(2) Determine the number of
software CSCIs in the aggregate; N

(3) For each
i^{th} CSCI, i = 1, 2, ...,
N, determine its criticality factor c_{i}. The lower the c_{i} the more
critical the CSCI.

(4) Determine t_{i}' the total
active time of the i^{th} CSCI, i = 1, 2, ..., N. Determine T, the
mission time of the aggregate.

(5) Compute the failure rate adjustment factor K:

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(6) Compute the allocated failure
rate goal of each CSCI

l_{i}
= l_{s} (c_{i }/ K)

(Dividing by K makes the allocated CSCI failure rates build up
to the aggregate failure rate goal).

__Example__:

Suppose a software aggregate consisting of three software CSCIs
is to be developed. Assume the failure rate goal of the aggregate is 0.002
failures per hour. Suppose that the mission time is 4 hours. Furthermore, the
criticality factors and the total active time of the software CSCIs are:

c 1 = 4 t’_{1}
= 2 hours

c 2 = 2
t’_{2} = 1
hour

c 3 = 1
t’_{3} = 2
hours

(Note: In this example, since c_{3} has the smallest
value, this indicates that the third CSCI of this software aggregate is the
most critical.)

Compute the adjustment factor
K: