Re: [SystemSafety] Does "reliable" mean "safe" and or "secure" or neither?

From: Dave Banham < >
Date: Sun, 24 Apr 2016 18:20:32 +0100

I strikes me that a lot of this discussion has revolved around various definitions that are either generic and abstract, or have a specific context without (generally) making it clear what that context is.

I do agree that we need good (if not sound) definitions for reliability, dependability, integrity, because there is considerable propensity to use them in everyday English with overlapped meaning. Moreover, I propose that there is subtly different application in these terms when considering physical things and non-physical things. This is further compounded by the behaviour of some physical things to be defined by non-physical things, as is the case in point of a computer based system whose behaviour is defined by the set of instructions in a so called program. A further complication also arises when emergent behaviours (intended/desired, undesired/unwanted but known, and undesired and unanticipated) also exist in the thing that we want to describe with such predicates. The main difference between physical and non-physical things is that physical things decay; they age and they wear, whereas non-physical things do not, but even here we have to be clear over the difference between information and the data that represents that information using a physical means.

What is reliable software? Answer: software that performs as required. (This of course assumes that by "software" we mean a program that is being executed by a machine.) Unreliable software does things that were not required of it, often with the connotation of undesired behaviour. The likelihood of software doing something that is not desired is a function of how well we understand the process of software execution (or indeed system use) that produces the undesired effect. When its is well understood then we can say we have a systematic defect of some sort, and when it is not at all understood then it will manifest as a statistical process. Our ability to shift from the latter to the former is all about the amount of effort we are prepared to invest in gaining that understanding.

Dave Banham

On 24/04/2016 17:12, paul_e.bennett_at_xxxxxx
> On 24/04/2016 at 4:55 PM, "Chris Hills" <safetyyork_at_xxxxxx >> Hi All
>> As I started this (and was only expecting a couple of replies
>> referring me
>> to the same definition) I thought I should chip in.
> We do need to have some general agreement about certain terms. The
> three main words that we should be absolutely clear on are:-
> Reliability
> Dependability
> Integrity
> A couple of other terms that should also be defined are:-
> Safe
> Secure
> however, I would expect the last two to be defined as part of the
> requirements definition of terms.
> I use the OED definitions for the most part (always one that seems
> to fit the bill).
> Regards
> Paul E. Bennett IEng MIET
> Systems Engineer

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systemsafety_at_xxxxxx Received on Sun Apr 24 2016 - 19:24:26 CEST

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