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

From: King, Martin (NNPPI) < >
Date: Fri, 6 Mar 2015 13:30:35 +0000


This message has been marked as UNCLASSIFIED by King, Martin (NNPPI)

Many safety shutdown systems will spend a considerable proportion of their time (90%+) in one of two plant states (operational and maintenance) with parameters that are quite limited in range. The two dominant states usually have parameter values that are quite disparate. Most of the remainder of the time is spent transitioning between these two states. In an ideal world the limited range of parameter values that will cause a shutdown will never occur - in practise they will normally occur extremely rarely over the life of the plant. Is this really the input value distribution that we want to test our equipment with?

Martin King
(My opinions etc, not necessarily those of my employer or colleagues!)  

-----Original Message-----
Sent: 06 March 2015 13:04
To: systemsafety_at_xxxxxx Subject: Re: [SystemSafety] Software reliability (or whatever you would prefer to call it)

I agree. That's why I added the point about explicit assumtions before using such measurements to predict the future.

There is usually a hidden assumption that the future input distribution will match that encountered during the measurement. But it's hard to justify having high confidence that such an assumption will prove correct.

Martyn

On 06/03/2015 12:32, Derek M Jones wrote:
> Martyn,
>
>> The company calculates some measure of the amount of usage before
>> failure. Call it MTBF.
>
> Amount of usage for a given input distribution.
>
> A complete reliability model has to include information on the
> software's input distribution.
>
> There is a growing body of empirical work that builds fault models
> based on reported faults over time. Nearly all of them suffer from
> the flaw of ignoring the input distribution (they also tend to ignore
> the fact that the software is changing over time, but that is another
> story).
>



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