From: Peter Bishop (pgb(at)adelard.com)
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 pgb(at)adelard.com, http://www.adelard.com/ pgb(at)csr.city.ac.uk, http://www.city.ac.uk/