10.8.1.2 FixedTime Sample
Plans
In fixedtime sample plans, the system performs consecutive
up/down cycles until a fixedtime T has elapsed. At this point, the test is
terminated and the system may be either up or down. In this case the test time
is fixed and the number of cycles is random.
Let A(T) = the observed availability at the end of the test.
The test procedure is

A(T) < A_{c}, then reject
H_{o} 
(10.117) 

A(T) ³ A_{c}, then accept
H_{o} 
(10.118) 
where the critical value A_{c} and test
time T are chosen so that the significance test satisfies the following
requirements on a and b.

P [ A(T) < A_{c}
A_{o} ,T
]
£ a 
(10.119) 

P [ A(T) ³ A_{c}
A_{1}, T ] £
b 
(10.120) 
If l_{p} is the upper P percentile of the standardized normal
distribution and time is in mean down time units, the test time to be used
is:
The critical value A_{c} is
The operating characteristic function is given by
where f is the standardized normal c.d.f.
Example 20: Exponential
Distribution
In this example use the same data as in the previous example.
A_{o} = 0.9, A1 = 0.8, m = n = 1 by the exponential assumption,
a = b  0.2.
Using Eq. (10.121),
T = 58.5 (Mean Down Time Units)
Using Eq. (10.122),
A_{c} = 0.856
The OC function is
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