Re: Critique of Ariane 5 paper (finally!)

[Thomas Beale <thomas_beale@invest.amp.com.au>]

Robert Dewar wrote:
> 
> There are many ways that one can use to determine if a program is reliable.
> For example, in some limited small cases, total proof of correctness may
> be a useful tool. Certainly the contstruction of such proofs can be aided
> by accurate assertions about the code, but such proofs do not (and cannot)
> depend critically on the presence of such assertions.

As an observer to this long-winded thread, can I suggest that you define
what
you mean by reliable? Software engineers such as myself often use the
term 
"correctness" to mean what you seem to be talking about above. I would 
define "reliability" as the ability of a system to execute correctly
over 
some time, and with "normal" inputs ("robustness" would be a measure of
a 
system handling pathological inputs). Reliability in my experience has 
often been specified by an mtbf - mean time between failures - figure.
So 
the aspect of quality we are interested in here is longevity of correct 
operation, not just correct function. So, when you say "reliability",
exactly
what do you mean?

- thomas beale