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<p>Why are you completely dismissing software reliablity? <br>
</p>
<p>Is it not the case that if you can tolerate a failure rate of
once in 1000 hours, 99% confidence through testing would take
about 200 days to demonstrate (so long as the test environment is
"sufficiently" like the future operating environment and you are
able to detaect every failure correctly)? <br>
</p>
<p>Of course, the testing would have to be repeated following a
change to the software, unless you have enough formality to show
that the change cannot affect reliability.</p>
<p>In specific circumstances, you can do better than this. Bev
Littlewood's published papers provide strong evidence and a rich
bibliography. Bev's paper on "How reliable is a program that has
never failed?" offers a useful rule-of-thumb: that aften n hours
of fault free operation, there is about 50% chance of a failure in
the following n hours (subject to some obvious constraints).<br>
</p>
<p>The difficulties rapidly escalate when you need 10^-4 or better
at >90% confidence. <br>
</p>
<p>Martyn<br>
</p>
<div class="moz-cite-prefix">On 14/09/2020 14:14, SPRIGGS, John J
wrote:<br>
</div>
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<p class="MsoNormal" style="margin-top:6.0pt"><span
style="font-family:"Roboto
Light";color:#365F91;mso-fareast-language:EN-US">In my
experience, if Software Reliability is mentioned at a
conference, at least one member of the audience will laugh,
and if it is mentioned in a work discussion, at least one
member of the group will get angry.<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-top:6.0pt"><span
style="font-family:"Roboto
Light";color:#365F91;mso-fareast-language:EN-US">Interestingly,
some of the same people who say it is impossible to quantify
software failure rates will set numerical requirements for
Software Availability – if you get one of those, ask the
Customer how (s)he wants you to demonstrate satisfaction of
the requirement.<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-top:6.0pt"><span
style="font-family:"Roboto
Light";color:#365F91;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-top:6.0pt"><span
style="font-family:"Roboto
Light";color:#365F91;mso-fareast-language:EN-US">John<o:p></o:p></span></p>
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1.0pt;padding:3.0pt 0cm 0cm 0cm">
<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span
lang="EN-US"> systemsafety
<a class="moz-txt-link-rfc2396E" href="mailto:systemsafety-bounces@lists.techfak.uni-bielefeld.de"><systemsafety-bounces@lists.techfak.uni-bielefeld.de></a>
<b>On Behalf Of </b>Derek M Jones<br>
<b>Sent:</b> 14 September 2020 12:54<br>
<b>To:</b> <a class="moz-txt-link-abbreviated" href="mailto:systemsafety@lists.techfak.uni-bielefeld.de">systemsafety@lists.techfak.uni-bielefeld.de</a><br>
<b>Subject:</b> [SystemSafety] What do we know about
software reliability?<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">All,<br>
<br>
What do we know about software reliability?<br>
<br>
The answer appears to be, not a lot:<br>
<a
href="http://shape-of-code.coding-guidelines.com/2020/09/13/learning-useful-stuff-from-the-reliability-chapter-of-my-book"
moz-do-not-send="true">http://shape-of-code.coding-guidelines.com/2020/09/13/learning-useful-stuff-from-the-reliability-chapter-of-my-book/</a><br>
<br>
-- <br>
Derek M. Jones Evidence-based software engineering<br>
tel: +44 (0)1252 520667
blog:shape-of-code.coding-guidelines.com<br>
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