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Lecture Software engineering (7/e): Chapter 24 – Ian Sommerville

After completing this unit, you should be able to: To explain how system reliability can be measured and how reliability growth models can be used for reliability prediction, to describe safety arguments and how these are used, to discuss the problems of safety assurance, to introduce safety cases and how these are used in safety validation. | Critical Systems Validation Objectives To explain how system reliability can be measured and how reliability growth models can be used for reliability prediction To describe safety arguments and how these are used To discuss the problems of safety assurance To introduce safety cases and how these are used in safety validation Topics covered Reliability validation Safety assurance Security assessment Safety and dependability cases Validation of critical systems The verification and validation costs for critical systems involves additional validation processes and analysis than for non-critical systems: The costs and consequences of failure are high so it is cheaper to find and remove faults than to pay for system failure; You may have to make a formal case to customers or to a regulator that the system meets its dependability requirements. This dependability case may require specific V & V activities to be carried out. Validation costs Because of the additional activities involved, the | Critical Systems Validation Objectives To explain how system reliability can be measured and how reliability growth models can be used for reliability prediction To describe safety arguments and how these are used To discuss the problems of safety assurance To introduce safety cases and how these are used in safety validation Topics covered Reliability validation Safety assurance Security assessment Safety and dependability cases Validation of critical systems The verification and validation costs for critical systems involves additional validation processes and analysis than for non-critical systems: The costs and consequences of failure are high so it is cheaper to find and remove faults than to pay for system failure; You may have to make a formal case to customers or to a regulator that the system meets its dependability requirements. This dependability case may require specific V & V activities to be carried out. Validation costs Because of the additional activities involved, the validation costs for critical systems are usually significantly higher than for non-critical systems. Normally, V & V costs take up more than 50% of the total system development costs. Reliability validation Reliability validation involves exercising the program to assess whether or not it has reached the required level of reliability. This cannot normally be included as part of a normal defect testing process because data for defect testing is (usually) atypical of actual usage data. Reliability measurement therefore requires a specially designed data set that replicates the pattern of inputs to be processed by the system. The reliability measurement process Reliability validation activities Establish the operational profile for the system. Construct test data reflecting the operational profile. Test the system and observe the number of failures and the times of these failures. Compute the reliability after a statistically significant number of failures have been observed. .