Inputs
What reliability needs
These are the required inputs for predicting reliability, the probability of success over a specific mission time.
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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 and growth period
Predicted from installed sites on the Required Inputs page
5
Months between major releases
Required Inputs tab — enter the years between releases
6
Predicted mission time
The average duration of a specific mission in operation — not the same as duty cycle. Example: a dishwasher cycle is 1 hour and runs daily; its monthly duty cycle is 30 hours, but its mission time is 1 hour.
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 years between releases — the time from the start of development to field deployment.
- The size estimates.
- The expected duty cycle per month of one typical installation.
- The predicted mission time in hours — the duration of one mission in operation (not duty cycle).
- The development factors, per the survey instructions.
Dashboard Prediction
Reading the reliability result
The Prediction sub-tab on the Dashboard shows the average calculation along with its lower and upper bounds, which follow from the bounds on the defect density and size estimates.
Availability prediction
Availability = average MTTF ÷ (MTSWR + average MTTCF)
Dashboard Predicted Growth
How reliability moves release to release
Reliability is calculated for every month after delivery. Because predictions only hold until the next major release, the best case is the prediction at the final month before release and the worst case is month one — e.g. with releases every 12 months, month 12 is best and month 1 is worst.
Reliability prediction, month i
Reliabilityi = Exp(−Mission Time / MTTCFi)
The subscript i indicates this prediction is for a particular point in time during operation. MTTCF is used as the input because only critical defects — those with no workaround — impact the mission’s probability of success while it’s ongoing.
Bounds
Lower and upper bounds follow the relative error on size estimations. At 50% relative error, the MTTCF upper bound sits 50% below the nominal failure-rate prediction and the lower bound sits 50% above it. Upper bounds are always the worst case; lower bounds are always the best case.
Reliability growth trends
The trend extends over eight years. Discovered defects and failure rate typically trend upward over that period, which pulls reliability predictions downward unless restore time can be reduced — software systems get larger over time, and every new release resets reliability growth as a function of the new code added. The software is never done.
That completes the prediction pipeline.
From size through to reliability, every stage feeds the next.