Re: [SystemSafety] Software reliability (or whatever you would prefer to call it)

From: Martyn Thomas < >
Date: Fri, 06 Mar 2015 10:51:03 +0000


Yes, it's astonishing how few people realise that you need a large enough, representative enough sample before you can apply statistics. Maybe it's the fault of our schools.

Martyn

On 06/03/2015 10:37, Nick Tudor wrote:
> Martyn
>
> Consider this then:
>
> The beta testing does not find any errors....according to your
> example, it must be 100% reliable.
>
> The fact that it did not hit the one undetected error in the code that
> would cause a system failure does not reflect in the supposed
> "reliability".
>
> Oh, then the software gets released and someone uses it in a manner
> such that it always hits the error - must be 0% reliable in the users
> view.
>
> Which would you like to pick?
>
> Nick Tudor
> Tudor Associates Ltd
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> On 6 March 2015 at 09:55, Martyn Thomas
> <martyn_at_xxxxxx > <mailto:martyn_at_xxxxxx >
> I'm puzzled by much of this discussion. Consider this common example:
>
> A company creates a software package and submits it for beta
> testing by
> a group of users. Assume that the package reports how often it is used
> and for how long, and the users report all errors they encounter.
> Assume
> there is a single instance of the software on a server that all the
> users use.
>
> The company corrects some of the errors that are reported.
>
> The company calculates some measure of the amount of usage before
> failure. Call it MTBF.
>
> The MTBF is observed to increase.
>
> What word shall we use to describe the property of the software
> that is
> increasing?
>
> I'd call it "reliability". If you would, too, then how can software
> reliability not exist?
>
> I don't mind if you want to use a different word to describe the
> property. Let's just agree one, do a global replace in the offending
> standards and move on ...
>
> ... to discussing a practical upper bound on the "reliability"
> that can
> be assessed in this way - and on the assumptions that should be made
> explicit before using any such assessment as a prediction of future
> performance.
>
> Martyn
>
>
>
>
>
>
>
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