SOFTWARE RELIABILITY MODELS COST

Prof. Dr. PANAGIOTIS SINIOROS
Technological Educational Institute of Piraeus Athens Greece
Prof. Dr. ION IVAN, 
Lect. Dr. MARIAN DÂRDALĂ, 
Academy of Economic Studies Bucharest

	ABSTRACT: Software qulity improvment, in the same way, have important results. For software reliability, an important characteristic of software quality, there are lots of models. This paper presents some of the models of reliability cost (linear model, nonlinear model).We estabilshed in a case of falling cost to one percent increase of software reliability. The problem of improvement the cost of reliability is very important for the development of indispensable software in the guiding proceeding of nuclear power station.
KEY WORDS:  Software Reliability, Cost Model

1. Introduction

The development of electrical production in the nuclear power station has low level of cost and pollution like advantage.
The reduction of accident risk needs to develop software with high level of reliability.
The power nuclear stations have special managerial conditions (the correlations between consumption and production, quality and structure of proccess).
The development of reliability software is possible using a modern technologies (structured analysis, object oriented analysis, Jackson system development, data flow oriented design).
The succes of technologies depends of CASE (Computer Aided Software Engineering) used.
The software reliability means a complexity and dimensin increased.
The software for nuclear power station particularry must have the greatest reliability. Such a software becomes useless because of its speed decrease while its cost an exponential increase.

2. The models of software reliability cost

There are created models for different steps of the software development cycle.
	The models quality depends the follows:
•	Work hypotesis
•	Factor numbers
•	Estimation methods
•	Stock of programs 
2.1. The estimated cost 
Considering set of programs {P1, P2, P3, …, Pn} extant, their complexity and expanses are measured. The model is:
	Ce=a*ebk,
where
	Ce – estimated cost,
	k – program complexity,
	a,b – coefficients.
	The a, b coefficients are estimated using the least squares method with data series having n terms.
	If we associate for a programm a tree structure of modules and complexity level is measured for each of module (Mi), the estimated cost is:
	
where:
	ki – the complexity level of module (Mi)
	m – number of modules
	A - assembly cost
	ai, bi – the estimated coefficients
	We can create complexity classes using the decomposed in modules.
	We estimated the ai,bi coefficients for each complexity class and we create a table with this coefficients.

2.1. The project cost
	
	This model contains the express computed during the proccess
	The model structure:

where:
	m – number of models
	chq – project expenses
	chpi – expenses for the i modul programming
	chdi – expenses for created of a test example of the i modul
chti – expenses for i modul testing
chai – expenses for the i modul assembly
ci – expenses for interface construction
ch – self documentation expenses
ct – expenses for global testing
cw – expenses for performed training
cc  – expenses for computer use
ca – archive expenses
cr – reproduction expenses
2.1. The effective cost
This cost is resulted from accounting:

where:
	Cf – effective cost
	P – stage number of software development cycle
	Cij – expense that was done in the i-th stages for the j-th stages.

This cost have to be corelated each other.
The first cost create an image about the resources level.
The second cost mobillize resource.
The third cost account the effective consumptions.

3. Reliability cost

The software reliability cost is part of effective cost and project cost and estimated cost. It’s very important to establish the reliability cost weight in total cost.
We define the methods for separate expenses on quality characteristic.
The reliability cost is computed like software complexity variation.

where:
	cf – software reliability cost
ccc – overagecost of quality characteristic
  - the unrealization of a characteristic in composes with a normated level ( - corectness,  - mentenability, - portability,  - reliability)

4. Conclusion

This theoretical results will be experimented in the future for create reliability software to help management in the nuclear power stations.

5. Acknowledgements

This paper continues our research activity. The work group encourage by T.E.I., A.S.E., I.C.I, National Commision for Research and High Education System Department.

Reference
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