# Direct and Indirect Measures, Reliability

## Direct and Indirect Measures :

A direct measure is obtained by applying measurement rules directly to the phenomenon of interest.For example, by using the specified counting rules, a software program’s “Line of Code” can be measured directly.

An indirect measure is obtained by combining direct measures.For example, number of “Function Points” is an indirect measure determined by counting a system’s inputs, outputs, queries, files, and interfaces

## Software Reliability

• IEEE 982.1-1988 defines software reliability as “The ability of a system or component to perform its required functions under stated conditions for a specified period of time.”
• Is the probability that a system will operate without failure under given environmental conditions for a specified period of time.
• Aims at fault-free performance of software systems

Software reliability engineering

• Engineering (applied science) discipline
• Measure, predict, manage reliability
• Statistical modeling
• Customer perspective:
• failures vs. faults
• meaningful time vs. development days
• customer operational profile

SRE Process

1. List Associated Systems
• Lists all systems associated with the product that must be tested independently and are of two types Base product & variations and Super-systems
1. Develop Operational Profiles
• An Operational profile is a Complete set of commands with their probabilities of occurrence
• System Testers and System engineers are included
1. Define “Just Right” Reliability
• Define what “failure” means for the product – Failure is defined as any Departure of system Behavior in execution from user needs – Failure intensity is the number of Failures per unit time
• Choose a common Measure for all failure intensities, either Failures per some natural unit or Failures per hour
• Set the total system Failure Intensity Objective (FIO) for each associated System – using Field data like Customer Satisfaction surveys related to measured failure intensity, – an analysis of competing products balancing among major quality characteristics users need.
• Finding a developed software failure intensity objective for any software you develop plus choosing among software reliability strategies to meet the developed software
1. Prepare for Test
• Specify the new Test cases for new operations of base product and variation for current release based on operation profile
• Prepares Test procedures to execute load tests
1. Execute Test
• determine an allocated Test time among Feature test, Load test, and Regression test, invoke test and identity system failure along with when they occur
• Feature tests – Interactions and Effects of the field environment minimized
• Load tests- execute Test cases simultaneously, with full interactions and all the effects of the Field environment
• Regression test-executes some or all feature tests and it is designed to reveal failures caused by faults introduced by program changes
1. Guiding Test
• Involves guiding the product’s system Test phase and Release
• Failure data is interpreted differently for software we are developing and software we acquire. We attempt to remove the faults that are causing Failures
• For developed software, we estimate the FI/FIO ratio from the times of failure events or the number of failures per time interval, using reliability estimation programs such as CASRE (Computer Aided Software Reliability estimation)

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Ans: Costs fall into two categories. There are cost associated with developing the systems and there are costs associated with a operating a system. view more..
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Ans: A direct measure is obtained by applying measurement rules directly to the phenomenon of interest.For example, by using the specified counting rules, a software program’s “Line of Code” can be measured directly. and sofware reliabity is .... view more..
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