Re: [sc] Origins of 10**-4 SW Failure Rate

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From: Peter Bishop (pgb(at)
Date: Fri 29 Nov 2002 - 08:44:31 GMT

On 29 Nov 2002, at 8:16, Peter B. Ladkin wrote:

>       ["Safeware" defn]Reliability is the probability that a piece of equipment
> or component wi
>       ll perform its intended function satisfactorily for a prescribed time and
>       under stipulated environmental conditions."
>       I was wrong here -- I should have written "specified function." 
> Otherwise,
>       it is undefined.
> It follows from the modified definition that the Ariane 501 software and
> hardware
> was completely reliable, even though the thing blew up after a few seconds of
> flight.

Does it follow?

The definition says "under stipulated conditions"
The flight trajectory for Ariane 5 meant that overflow was possible
- but was not possible if it followed an Ariane 4 trajectory.
So either it was unreliable under Ariane 5 conditions
Or reliable under Ariane 4 conditions

This question then would be "what conditions where stipulated?"
If the wrong condition were stipulated - or the conditions omitted
it looks like a flaw in the initial requirements.
Peter Bishop 
Principal Consultant, Adelard
Professor, Centre for Software Reliability, City University
Drysdale Building, 10 Northampton Square, London, EC1V 0HB
Tel: +44-20-7490-9467, Fax: +44-20-7490-9451

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