Software Quality ManagementUnit – 3 

Roy Antony Arnold GAsst Prof /CSEAsst. Prof./CSE

• Software reliability models are• Software reliability models are

when it is available to thecustomers.

• The criterion variable under study is the number of ydefects in specified time intervals (weeks, months, etc.), or the time between failures.

• Such an estimate is important for two reasons:– (1) It is an objectiveobjective statementstatement of the qualityquality ofof thethe

d td tproductproduct– (2) It is a resource planning tool for the softwaremaintenance phasemaintenance phase.

• Reliability models can be broadly classified into two categories:Reliability models can be broadly classified into two categories:and (Conte et al., 1986).

• A static model uses other attributes of the project or programmodules to estimate the number of defects in the softwaremodules to estimate the number of defects in the software.General Form :

The number of defects (y) is dependant on the attributes (x) of theThe number of defects (y) is dependant on the attributes (x) of the product and the process by which it is produced,  plus some error (e) due to unknowns which inherently exist.

• A dynamic model usually based on statistical distributions usesA dynamic model, usually based on statistical distributions, usesthe current development defect patterns to estimate end‐product reliability.

• Dynamic Models are classified in two categoriesDynamic Models are classified in two categories– those that model the entire development process (Rayleigh Model)– those that model the back‐end testing phase (Exponential Model 

and Reliability Growth Models)and Reliability Growth Models)

• The Rayleigh model is a parametric model• The Rayleigh model is a parametric modelin the sense that it is based on a specific

i i l di ib i i d istatistical distribution. It is a dynamicreliability model.

• When the parameters of the statisticaldistribution are estimated based on thedistribution are estimated based on thedata from a software project, projectionsb t th d f t t f th j t babout the defect rate of the project can bemade based on the model.

• The Rayleigh model is a member of the family of the• The Rayleigh model is a member of the family of the.

• One of its marked characteristics is that the tail of itsOne of its marked characteristics is that the tail of itsprobability density function approaches zeroasymptotically, but never reaches it.

• Weibull distributions are used for predicting reliability and probability distribution

• Two standard functions for graphing Weibull

• Rayleigh is a special case of the WeibullRayleigh is a special case of the Weibullwhere the shape parameter (m) equals 2:

• The formulas represent a standard distribution.

• The total area under the curve is 1.

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The defect rate observed during the development process isg p ppositively correlated with the defect rate in the field. (Fig.)

Assuming the defect removal effectiveness remains unchanged, thenh h ( d f ) d d l h ha higher curve (more defects) during development means a higher

defect injection rate and hence a higher field defect rate.

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Given the same error injection rate if more defects areGiven the same error injection rate, if more defects arediscovered and removed earlier then fewer will remain inlater stages and the field quality will be better.g q y– In the fig. the areas under the curves are the same but the curves 

peak at varying points. Curves that peak earlier have smaller areas at the tail the GA phaseat the tail, the GA phase.

In short “Do it right the first time ”In short, “Do it right the first time.”This means that if each step of thedevelopment process is executed properlydevelopment process is executed properlywith minimum errors, the end product'squality will be good.

Given the same error injection rate if more defects areGiven the same error injection rate, if more defects arediscovered and removed earlier then fewer will remain inlater stages and the field quality will be better.g q y– In the fig. the areas under the curves are the same but the curves 

peak at varying points. Curves that peak earlier have smaller areas at the tail the GA phaseat the tail, the GA phase.

• Most statistical software packages supportMost statistical software packages support Weibull Distributions.

l b d l d d h• Applications can be developed due to the clearly defined algorithms for Weibull.

• COTS (Commercial Off The Shelf) products can also be used:can also be used:

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• Accuracy of model estimatesAccuracy of model estimates.• Input data must be accurate and reliable.• To establish high Predictive Validity,To establish high Predictive Validity,

and empirical validity must beestablishedestablished.

• The validity of software reliability modelsA certain model may work well for a. A certain model may work well for a

specific organization or development structure, butnot for others.

• No universally good software reliability modelexists.

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• High‐level Design Review (I0), Low‐level Design Review (I1), Code g g ( ), g ( ),Inspection (I2), Unit Test (UT), Component Test (CT), System Test (ST), and General Availability Phase (GA)