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OCR for page 81

Appendix A
Further Details Concerning
the Bearing Cage Example
_ ' n Chapter 3, an overview of a frequentist analysis of the reliability of a
bearing cage was provided, starting with the analysis when the two
~ ~ Weibull parameters, 77 and A, are both estimated, and then demon-
strating the benefits from using other sources of information to fix p. This
was followed by a Bayesian analysis in which other sources of information
were used to provide a prior for p.
With respect to the frequentist analysis, this appendix provides addi-
tional probability plots for the bearing cage data, fixing ~ at the values 1.5,
2, and 3. These plots show the lack of support for the assertion that B 10 is
8,000 hours.
With respect to the Bayesian analysis, this appendix provides two plots,
the first of the prior distribution for B10 and A, and the second for the
posterior distribution for B10 and p. Both plots have the data likelihood
superimposed. It is clear that the data likelihood has moved the posterior to
be more consistent with it than the prior.
Figures A-1, A-2, and A-3 show probability plots with the Weibull
shape parameter ~ fixed at 1.5, 2, and 3, respectively. Although Figure A-1
may allow for a little optimism, the overall conclusion suggested by these
figures is that the bearing cage is, most likely, not meeting its reliability
goal.
81

OCR for page 81

82
.2
1 -
05 -
.02 -
.01 -
.005 -
.003
.001 -
.0005 -
APPENDIXA
_,, . ~ -
77= 28982
,0= 1.5
200 500 1,000 2,000 5,000 10,000
Hours of Service
FIGURE A-1 Weibull probability plot of the bearing cage failure data showing the
maximum likelihood estimate of fraction failing with fixed ,0 = 1.5, the reliability target,
and approximate confidence limits. SOURCE: Meeker and Escobar (1998~.
.98 -
9
.7
.5
. _
ct. .05
~ .02
.O .01
cd .005
IL .003
.001
.0005
0002
.0001
.00005
.00003
.,
.,
by= 12320
~=2
1 1 1 · 1 1 1
200 500
1,000 2,000
Hours of Service
5,000 1 0,000
FIGURE A-2 Weibull probability plot of the bearing cage failure data showing the
maximum likelihood estimate of fraction failing with fixed ,0 = 2, the reliability target,
and approximate confidence limits. SOURCE: Meeker and Escobar (1998~.

OCR for page 81

THE BEARING CA GE EXAMPLE
.98
9-
.7
5 -
. _
IL .02
~ a`
o .u1
C.) .005
~ .003
IL
83
77= 5409
p=3
I ~ I ~ I ~ ~ T ~ ~
200 500 1,000 2,000
Hours of Service
5,000 1 0,000
FIGURE A-3 Weibull probability plot of the bearing cage failure data showing the
maximum likelihood estimate of the fraction failing with fixed ,0 = 3, the reliability
target, and approximate confidence limits. SOURCE: Meeker and Escobar (1998~.
BAYES ANALYSIS
Figure A-4 shows a sample of points from the joint prior distribution
of ,0 and B 10 superimposed on a graph of the relative likelihood contours.
The truth is expected to lie at the intersection of the prior and the likeli-
hood.
A sample from the posterior can be obtained by filtering the sample
from the prior distribution. This can be done by selecting points in the
prior distribution with a probability equal to the likelihood contour going
through the point. Points are run through this filter until a sufficient num-
ber of posterior points are obtained to estimate the posterior distribution
(for two parameters, something on the order of 6,000 to 10,000 is suffi-
cient).
Figure A-5 shows a sample of points from the joint posterior distribu-
tion, again with the likelihood contours.

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84
APPENDIXA
4.0 -
3.5 -
3.0 ,
2.5]
2.0 -
1.5 -
1.0 -
0.5 '
1~\
· .
.
· ~
· :,,.; ';~1,~,,:~, ,
.
.
.
~1
~1
500 1,000 2,000 5,000 10,000 20,000
B10
FIGURE A-4 Sample points from the jointpriordistribution of B10 of bearing cage life
and the Weibull ~ with the data likelihood superimposed. SOURCE: Meeker and
Escobar (1998~.
4.0
3.5
3.0
2.5 -
1 1 ~ 1
~~ ,,, Jo:" ,
2.0 -
1.5 -
1.0 -
0.5 · 1 1 1 1 1 1
\
it\
.
~~\
~ U. 1
~1
500 1,000 2,000 5,000
B10
1 o,ooo 20,000
FIGURE A-5 Sample points from the jointposteriordistribution of B10 of bearing cage
life and the Weibull ~ with the data likelihood superimposed. SOURCE: Meeker and
Escobar (1998~.