[aadl]: [AADL]: Question about error model events distribution

[Myron J Hecht]

Hello everyone

>Are you sure that it [Syntax of distributions] is formalized enough (e.g. 
in the standard text) to be used by instruments or formal analysis?

Probability distributions used in stochastic analysis of systems for 
reliability and safety are quite well defined.  Whether they are properly 
used is a function of the external calculation/simulation tool and the 
code that provides the interface from the EMV2 section of the AADL model 
to that tool. 

Regards


Myron Hecht
Sr. Project Leader
The Aerospace Corporation
myron.hecht at aero.org
310-336-3521



From:   Viet Yen Nguyen <nguyenvietyen at gmail.com>
To:     Denis Buzdalov <buzdalov at ispras.ru>, 
Cc:     Sergey Zelenov <zelenov at ispras.ru>, sae-aadl-users 
<sae-aadl-users at lists.sei.cmu.edu>
Date:   10/22/2014 05:38 AM
Subject:        Re: [aadl]: Question about error model events distribution
Sent by:        
sae-aadl-users-bounces+myron.j.hecht=aero.org at lists.sei.cmu.edu



Hi Denis,

1) Regarding time periods: the time period is implicit to the \lambda 
parameter. You of course have to make sure that all \lambda parameters in 
the model are based on the same time period. See also the definition of 
the Poisson distribution on Wikipedia.

2) Regarding the syntax and use of the distributions, I don't have a copy 
of the AADL Error Annex here at hand (the copy I used to have belonged to 
the my previous employer). I cannot therefore lookup and confirm for you 
whether the current text is formal enough on this. Perhaps somebody on the 
mailinglist who is more intimate with the AADL Error Annex can jump in 
here. 

3) Theoretically, any probability distribution is oblivious towards its 
use. Its semantics are decoupled from the meaning of the random variable 
that is being probabilistically distributed. So far we've been taking 
about amount of occurrences, waiting times and decision answers as random 
variables. The choice of distribution for a random variable therefore 
depends on whether the distribution's characteristics match your intended 
semantics (i.e. the real world). 

Viet Yen

On Wed, Oct 22, 2014 at 2:03 PM, Denis Buzdalov <buzdalov at ispras.ru> 
wrote:
Hi Viet Yen,

Thank you for your response. But, you know, I still have questions to
your answer.

> Let us translate that to practical terms. The sample 1 means that the
> error event occurs 1 time (within the timeframe according to the
> \lambda parameter of the Poisson distribution). The sample 100 means
> that the error event occurs 100 times.

The first question is what period of time event occurrence is
considered? This distribution shows the probability of n occurrences of
event during what period of time?

> Given the \lambda parameter,
> the Poisson distribution assigns a probability to that, e.g. the
> probability that the error event occurs 100 times.

The second question is am I right that you mean that \lambda parameter
can be set through the 'ProbabilityValue' record part of the
'EMV2::OccurrenceDistribution' property value?

If the answer is 'yes', then another question rises: how would you
set parameters for multiple-parameterized distributions (like the Normal
distribution)?

If the answer to the second question is 'no' then I would ask how would
you set the \lambda parameter?

> Another interesting distribution is the exponential one. It's
> continuous and spawns a probability distribution over the waiting
> time before the error event happened. For example, the probability
> that you're 100 time units in the OK state before the error event
> happens.

This part of your answer makes me messed up completely.

Am I right that you mean that the semantics of distribution setting
depends completely on the distribution type: you have
- the probability of occurrence in one case,
- expected count of occurrences in the other case and
- expected time on the third case?

Are you sure that it is formalized enough (e.g. in the standard text)
to be used by instruments or formal analysis?

--
Denis Buzdalov
Software Engineering Department, ISPRAS

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