Skip to content

Exercises related to Robust Development Methodologies - Formal

Availability, Reliability and Mean Time To/Between…

Exercice 1

In safety (and elsewhere), the following terms are used very often:

  • Reliability
    the ability of an equipment to function without failure More details
  • Availability
    the probability that an item will operate satisfactorily at a given point in time More details
  • Mean Time Before Failure (MTBF)
    the predicted elapsed time between inherent failures of a mechanical or electronic - repairable - system during normal system operation
  • Mean Time To Failure (MTTF)
    denotes the expected time to failure for a non-repairable system
  • Mean Time To Recovery (MTTR)
    the average time that a device will take to recover from any failure
  • Bathtub-Shaped Lifetimes
    if enough units from a given population are observed operating and failing over time, it is relatively easy to compute week-by-week (or month-by-month) estimates of the failure rate
    More details

Questions to you

These terms may give a feel of their meaning that may be misleading. So, please answer the following questions:

  1. Given a product that has a MTBF of 100 years, can you develop how many devices will still be working after 100 years given the following reliability formula?

    \(R(t) = e^{-t/MTBF}\)

    What does this mean for a single product and its chance of working after that period?

    (note: the above is valid only if one takes the bathtub curve model in its constant failure rate phase)
  2. Assuming your product needs to achieve 99,9999% (aka “six nines”), what are the maximum acceptable downtime per year? How do you calculate this?

    (hint: the answer is in one of the links above)

Going beyond

Side Note

Von Neumann, mostly famous for his computer architecture work, also worked extensively on creating reliable systems out of unreliable components (paper)

Standardization, Certification and Processes

Exercice 2

Given the picture below, we could state that:

  1. Part 1: phases 1–5 address analysis
  2. Part 2: phases 6–13 address realisation
  3. Part 3: phases 14–16 address operation

Questions to you

  1. Of the 3 main phases stated above, state what are the focal point of the present course.
  2. What does “Back to appropriate overall safety lifecycle phase” mean in your opinion? (Arrow leaving block 15 - “Overall modification and retrofit”)

MISRA

Exercice 3

We have discussed the importance of different aspects of MISRA. Read the following guideline Achieving compliance with MISRA Coding Guidelines and respond to the questions below.

Questions to you

  1. Why is it important to train the staff?
  2. In the context of MISRA, what tools are comprised under “Tool Management”?

Going beyond/References