[SystemSafety] Difference between software reliability and astrology

[Prof. Dr. Peter Bernard Ladkin]

On 2024-08-14 17:42 , Derek M Jones wrote:
> Peter,
>
>>  He doubted that on-demand functions can be said to have an average probability of failure on 
>> demand (PFD_[avg]); and he doubted that continuously-operating functions can be said to have a 
>> Mean Time To Failure 
>
> When talking about software, he is right.

When talking about on-demand software functions, he is demonstrably wrong, as I said.

When talking about continuously-operating functions, do you have any coherent argument that there is 
no MTTF?

> If known faults don't get fixed, then data on rate of fault experiences
> can be used to estimate a value for Mean Time To Failure.

..... but it sounds as if you don't, if you think you can estimate a value for it.

It looks to me as though you are contradicting yourself by suggesting that M is right to suggest 
that continuously-operating functions may have no MTTF, but then suggesting how you can estimate it. 
What on earth can you mean?

>> I did try deconstructing continuous SW, say a feedback control system, as (a) a rapid (hundreds 
>> of Hz) polling/sampling routine, which then calls (b) an on-demand routine; and then (c) 
>> considering (a) as a very rapid Bernoulli process. That gives me the desired result, but it is 
>> wrong, because you can't consider (a) to be a Bernoulli process because it's not 
>
> Bernoulli process might work well enough for hardware, but the
> characteristics of software are very different.

Read https://rvs-bi.de/publications/books/RVS-Bk-17-01/Ch01-SoftwareUrnModelFailure.pdf

The essay stems from 2015.

PBL

Prof. Dr. Peter Bernard Ladkin
Causalis Limited/Causalis IngenieurGmbH, Bielefeld, Germany
Tel: +49 (0)521 3 29 31 00