# Presentation: 192948

## Integrating Software into PRAPresented byCarol SmidtsCenter for Reliability EngineeringUniversity of Maryland, College ParkJuly 20, 2004

## Statement of the Problem

- Probabilistic Risk Assessment (PRA) is a methodology to assess the probability of failure or success of a system.
- Software plays an increasing role in modern safety critical systems. Unfortunately current PRA practice neglects the contributions of software to the risk of failure of the mission.
- The objective of our research is to extend the current PRA methodology to integrate the impact of software failures in the risk assessment process.

## The PRA Process

**	****PRA is the process designed to answer four basic questions:**
- What can go wrong?
- What are the consequences of things going wrong?
- How likely are these undesirable consequences?
- How confident are we about our answers to the above questions?

## Framework

## Importance/Benefits

- The research will contribute to the evaluation of risks to NASA missions and more specifically it will contribute to the understanding of the impact of software on mission success.
- It will also quantitatively point to the software sub-systems or modules that contribute most to safety and need to be developed with the greatest care.
- It will help increase software safety by an active consideration of the contexts within which software can be used.
- It will allow consideration of software in the trade studies carried out for upgrades.
- It will allow better appraisal of software development issues since software development data may be used in the quantification process. This return of experience may help in developing procedures to enhance software development activities.

## Relevance to NASA

- 	Since PRA techniques are intended for all NASA missions, the research has the potential to benefit all NASA centers. Furthermore, it is expected that the Space Shuttle, Space Station and advanced space missions and related software assurance activities will directly benefit from the case study.

## Accomplishments

- We developed a taxonomy of software related-failures which accounts for : input failures, output failures, failures of the software itself, multiple interaction failures, support failures and environmental conditions. (ISSRE 2002)
- The software related failure mode taxonomy was validated by JSC using NASA data. The NASA validation indicates that our taxonomy is complete, consistent, and applicable. The repeatability is adequate and can be further increased if documentation is available and a rigorous process of classification is followed. (PSAM7/ESREL 2004)

## Accomplishments

- We proposed an initial version of the test-based approach to integrate software into PRA and applied this approach to a small control system, PACS. This approach includes identification of software functions to be modeled in the PRA, modeling of the software contributions in the ESD and Fault Tree. The approach also introduces the concepts of input tree and output tree and proposes a quantification strategy that uses a software safety testing technique. (PSAM7/ESREL 2004)
- We conducted an expert opinion elicitation using BBN techniques to study the causal factors of the different failure modes.

## Accomplishments

- We performed an initial study of scalability. This study decomposes the level of effort into modeling time (time to build the finite state machine), test case generation time and test execution time. The results show that:
- Modeling time can be calculated by using COCOMO II;
- Test generation time in full cover is a function of the size of the model;
- Test execution time is a linear function of the number of the input/output, numbers of check points and the waiting time for responses.

## Accomplishments

- We conducted a study of fault propagation mechanisms to understand how the particular failure modes identified lead to failure. We developed an approach to estimate the probability of fault propagation in the sequence structure with one input variable for the wrong input value failure mode.

## Further Research and Application

- Perform a complete study of support and output failures.
- Continue the propagation study.
- Finalize scalability.
- Continue applying the methodology to CM1
- Collaborate with JSC to validate the test-based approach on a large scale application.