Expected Failure Time Plot

When a reliability life test is planned it is useful to visualize the expected outcome of the experiment. The Expected Failure Time Plot (introduced by ReliaSoft in Weibull++ 8)provides such visual.

Background & Calculations

Using the cumulative binomial, for a defined sample size, one can compute a rank (Median Rank if at 50% probability) for each ordered failure.

As an example and for a sample size of 6 the 5%, 50% and 95% ranks would be as follows:

Table 1: 5%, 50% and 95% Ranks for a sample size of 6. 

Order Number	5%	50%	95%
1	0.85%	10.91%	39.30%
2	6.29%	26.45%	58.18%
3	15.32%	42.14%	72.87%
4	27.13%	57.86%	84.68%
5	41.82%	73.55%	93.71%
6	60.70%	89.09%	99.15%

Furthermore, consider that for the units to be tested the underlying reliability model assumption is given by a Weibull distribution with β = 2, and η = 100 hr.

Then the median time to failure of the first unit on test can be determined by solving the Weibull reliability equation for t, at each probability,

or

[math]\displaystyle{ R(t)=e^{\big({t \over \eta}\big)^\beta} }[/math]

then for 0.85%,

[math]\displaystyle{ 1-0.0085=e^{\big({t \over 100}\big)^2} }[/math]

and so forths as shown in the table below:

9.25
25.48
40.77
56.26
73.60
96.64

33.99
55.42
73.97
92.96
115.33
148.84

70.66
93.37
114.21
136.98
166.34
218.32