Prediction Inputs
What failure rate and MTTF need
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Input
Tab
1
Predict size
Required inputs, under the Survey tab
2
Predict defect density
All Survey tabs except Failure Modes
3
Duty cycle, fraction of defects affecting availability, installed sites, release schedule
Required inputs, under the Survey tab
4
Growth rate (Q) and growth period (TF)
Predicted from installed sites on the Required Inputs page
5
Months between major releases
Results only display for months until the next major release, since new code resets reliability growth
Go to the Required Inputs page and enter the following:
- Typical fraction of defects found in operation that affect availability. Determine this from past failures on a similar system, or from testing — e.g. 1,000 defects were found testing the last release, and 10 affected availability with no workaround, a fraction of 0.01.
- The number of installed sites that most closely matches how many installations of this version are planned.
- The schedule time in years for this release — the time from the start of development to field deployment.
- The size estimates.
- The expected usage hours per month of one typical installation.
- The development factors, per the survey instructions.
Dashboard Prediction
Average metrics for the release cycle
Average failure rate & MTTF
Average Failure Rate = Total defects predicted in operation / (Average usage hours per month × Schedule years for release × 12)
Average MTTF = 1 / Average Failure Rate
Average Critical Failure Rate = Total critical defects predicted in operation / (Total usage hours per month × Schedule years for release × 12)
Average MTTCF = 1 / Critical Failure Rate
Average MTTF = 1 / Average Failure Rate
Average Critical Failure Rate = Total critical defects predicted in operation / (Total usage hours per month × Schedule years for release × 12)
Average MTTCF = 1 / Critical Failure Rate
Total usage hours per month and schedule years for release are entered on the Required Inputs tab under the Survey tab.
Dashboard Predicted Growth
Point-in-time metrics, in test or operation
Predicted MTTFi
= T / (N(Exp(−Q / TF × T1) − Exp(−Q / TF × T2)))
Predicted MTTCFi
= T / (NC(Exp(−Q / TF × T1) − Exp(−Q / TF × T2)))
Predicted Failure Ratei
= (N(Exp(−Q / TF × T1) − Exp(−Q / TF × T2))) / T
Predicted Critical Failure Ratei
= (NC(Exp(−Q / TF × T1) − Exp(−Q / TF × T2))) / T
Symbol
Meaning
T
Duty cycle in month i, in hours, for the software
Q
Growth rate — see the defect profile page
TF
Growth period — see the defect profile page
T1
Beginning of the interval being solved for — e.g. for month 3, T1 = 2.
T2
End of the interval being solved for — e.g. in the previous example, T2 = 3.
N
Number of inherent defects delivered — the result of step 4 of the prediction process. For testing metrics, this is predicted defects to be found in testing.
NC
Number of critical defects delivered — the result of step 4 of the prediction process. For testing metrics, this is predicted critical defects to be found in testing.
Bounds & the subscript i
The subscript i indicates this prediction is for a particular point in time during operation, between T1 and T2. Lower and upper bounds follow the relative error on size estimations: at 50% relative error, the MTTCF upper bound is 50% lower than nominal and the lower bound is 50% higher; failure rate and critical failure rate move the other way, with upper bounds 50% higher than nominal and lower bounds 50% lower. Upper bounds are always the worst case; lower bounds are always the best case.
Reliability growth trends
The trend extends over eight years, typically showing an increasing trend of discovered defects and failure rate, and a corresponding decrease in MTTF. This is because software systems get bigger over time — every new release resets reliability growth as a function of the new code added. The software is never done.
Continue to the next stage of the prediction.
MTTCF feeds directly into the availability calculation.