3-12
Quality
Statistical Reliability
Statistical Reliability
The sections that follow explain the Annualized Failure Rate (AFR) and the Mean Time
Between Failure (MTBF) computations.
Annualized Failure Rate Projections
Estimated AFR is based on a parts count and the failure rates of those parts in similar prod-
ucts. The AFR is estimated from the sum of the failure rates of the components’ assemblies.
A key assumption is that AFR is constant beyond the early life of the product and before
long-term wear out.
Projected Failure Rate is the mature AFR, estimated from the failure rates of the components
of the product. The "Estimated Long-Term Annualized Failure Rate" is a future projection of
an average realistic failure rate beyond the typical infant failure period that occurs in the first
few months of use and before long-term wear out, which occurs after many years of use.
Mean Time Between Failure Computations
The MTBF is the reciprocal of the AFR. MTBF uses the following two units of time:
•
Annualized MTBF (A-MTBF), the time units are years, or annually.
•
Hourly MTBF (H-MTBF), the time unit is the hour.
This is derived by factoring out the AFR's annualized percentage, then finding its reciprocal.
A resultant A-MTBF term is in years.
Table 3-26
MTBF Example
Example
AFR
Model 744 VME board computer with graphics
4.8%/year
Add one 32MB RAM card
0.72%/year
Total
5.5%/year
1
0.055 / 8766 hours
= 159,381 hours
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