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

From: Nick Tudor < >
Date: Fri, 6 Mar 2015 10:55:44 +0000


Martyn - Yes - it's astonishing how often the one in a million chance seems to happen 9 times out of 10....especially with software systems.

Nick Tudor
Tudor Associates Ltd
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www.tudorassoc.com

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On 6 March 2015 at 10:51, Martyn Thomas <martyn_at_xxxxxx wrote:

> 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
> Mobile: +44(0)7412 074654
> www.tudorassoc.com
>
> *77 Barnards Green Road*
> *Malvern*
> *Worcestershire*
> *WR14 3LR*
> * Company No. 07642673*
> *VAT No:116495996*
>
> *www.aeronautique-associates.com
> <http://www.aeronautique-associates.com>*
>
> On 6 March 2015 at 09:55, Martyn Thomas <martyn_at_xxxxxx > wrote:
>
>> 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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systemsafety_at_xxxxxx Received on Fri Mar 06 2015 - 11:55:48 CET

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