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<div class="moz-cite-prefix">On 15/09/2020 11:28, Peter Bernard
Ladkin wrote:<br>
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cite="mid:dc2323d2-4f5c-25ac-1154-6091610a5692@causalis.com">
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On 2020-09-15 11:21 , Coq, Thierry wrote:
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<pre class="moz-quote-pre" wrap="">...Ariane A501 flight has disproved any probabilistic approach on software that would not
start with a frequency of failure of 1.
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<p>I have to disagree. It does not disprove the probabilistic
approach. The point of the tests is to set an upper bound on the
likelihood of failure.</p>
Statistical test should use a realistic profile for the tests. So
for Ariane V, if you ran the system in an iron bird simulator with a
Ariane V launch profile, it would indeed crash first time - so the
bound is effectively 1 given the test result.<br>
<p>From this we would conclude it is highly unlikely to have a crash
probability better than 10-2 (or some such desired target).</p>
<p>If we fixed the bug and and then ran 300 simulated launches
without failure we conclude it would meet the target with 95%
confidence.</p>
<p>It does not mean the system is bug-free, just that any bugs that
may be present are unlikely to be activated for a realistic
profile.</p>
<p>If you fixed the original bug and and did not do the 300 tests,
what confidence would you have that the next launch of the Ariane
V would succeed?<br>
- might be cheaper to do the tests rather than finding out the
hard way.</p>
<p>Peter<br>
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cite="mid:dc2323d2-4f5c-25ac-1154-6091610a5692@causalis.com">
<pre class="moz-quote-pre" wrap="">I have no idea what this sentence means.
I think everybody would agree that the environmental circumstances leading to failure of the Ariane
FLight 501 control system were certain to arise during launch. I don't see what probability has to
do with it.
Whatever people's inclinations to statistical evaluations of SW, we are about to be deluged by it.
The DLNN assistance functions in the automation of road vehicles can at present only be justified by
an assessment of their in-service behaviour. There are going to be numerical requirements on their
dependability and vendors will be providing argument that those numerical requirements are fulfilled.
The statisticians know well how much evidence has to be produced in order to derive conclusions on
reliability to an appropriate level of confidence. (See, for example, Peter Bishop's comment here on
the Tempe accident.) It is a lot higher than what, as far as I can tell, the vendors of such
equipment are likely be able to produce. What's going to happen?
PBL
Prof. Peter Bernard Ladkin, Bielefeld, Germany
Styelfy Bleibgsnd
Tel+msg +49 (0)521 880 7319 <a class="moz-txt-link-abbreviated" href="http://www.rvs-bi.de">www.rvs-bi.de</a>
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<pre class="moz-signature" cols="72">--
Peter Bishop
Chief Scientist
Adelard LLP
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