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The Dynamics of Software Project

Staffing: A System Dynamics

Based Simulation Approach

IEEE Transactions on Software Engineering(TSE, 1989) TAREK K. ABDEL-HAMID

Park, Ji Hun 2010.5.10

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Contents

Introduction

Background

Overall approach

Conclusion

Discussion

2 / 24 KAIST SE LAB 2010

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Introduction

Problem statement

 In 1989, people issues have gained recognition

 There is lack of researches on the whole life cycle of software development as an integral

 It is remarkably difficult to test hypothesis in software engineering

3 / 24

In this paper,

 Suggests system dynamics simulation of software development process

 Integral model to describe software development process

 Formal method to verify hypothesis in software engineering

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System dynamics model

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KAIST SE LAB 2010

Background (1/3)

 Approach to understanding the behavior of complex

system over time

 Looking at things as a whole

 Simulation large system with feedback and loop

LEVEL

Rate

(5)

System dynamics model (cont‟d)

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KAIST SE LAB 2010

Level : accumulation over time of flows

Rate : The flows increasing and decreasing a level

Auxiliary : Intangible, additional algebraic computations

LEVEL Rate Source Sink Rate Input Input Output Auxiliary

Background (2/3)

(6)

System dynamics model (cont‟d)

6 / 24 KAIST SE LAB 2010 Influencing Variable that is on Other Diagram Variable Being Influenced on Other Diagram Information Flows Other Flows

(e.g., People, Software)

Flows

Constant

Variable cross-reference

Constant

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Integrative System Dynamics Model of Software Development

An integrative perspective

 Including management-type functions and software production-type activities

Four subsystems

7 / 24 Human resource management Software Production Controlling Planning Work force needed Work force available Effort remaining Progress status Tasks completed Schedule

(8)

The human resource management subsystem(1/8) NEWLY HIRED WORKFORCE EXPERIENCED WORKFORCE HIRING RATE HIRING DELAY TRANSFER DELAY TRANSFER DELAY AVEARAGE ASSIMILATION DELAY MOST NEW HIRES PER FT EXP STAFF AVEARGE EMPLOYMENT TIME WORKFORCE ASSIMILATION RATE QUIT RATE FTE EXPERIENCED WORKFORCE CEILING ON NEW HIRES CEILING ON TOTAL WORKFORCE WF LEVEL SOUGHT WORKFORCE GAP TOTAL WORKFORCE NEWLY HIRED TRANFER RATE EXPERIENCED TRANFER RATE WORKFORCE LEVEL NEEDED

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Separating the workforce into two categories

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The human resource management subsystem(2/8)

Experienced Newly hired It takes 15% ~ 25% of an experienced employee’s time  Training process

• Training of new comers is usually carried by experienced staffs

 “Orientation phase”

• For newly hired and

transferred from other project members

• Both technical as well as social dimensions

• They are less than fully productive

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Workforce level needed

 “Indicated workforce level”

• Workforce level which is necessary to complete the project on schedule

 Workforce level needed =

(Indicated workforce level) * (WCWF)

+ (Current workforce) * (1 – WCWF)

 WCWF : Willingness to Change WorkForce

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The human resource management subsystem(3/8)

Indicated workforce level Current workforce gap Weight factor WCWF : 1-WCWF

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Willingness to Change WorkForce(WCWF)

 WCWF-1

• Capturing the pressures for workforce stability

• Increases when „Time remaining / Time parameter‟ increases

• Time parameter = Hiring delay + Average assimilation delay

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The human resource management subsystem(4/8)

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Willingness to Change WorkForce(WCWF) (cont‟d)

 WCWF-2

• In organizations such as NASA, serious schedule slippages could not be tolerated

• Increases when „Scheduled completion date / Maximum tolerable completion date increases

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The human resource management subsystem(5/8)

(13)

Willingness to Change WorkForce(WCWF) (cont‟d)

 WCWF = MAX ( WCWF-1 , WCWF-2)

• When the early phase of the project, WCWF-2 would be zero

• When the project reaches to the end, usually WCWF is dominated by WCWF-2

 WCWF is an expression of a policy for managing

• Also, the shapes of the WCWF policy curves can be derived on the basis of historical project records

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The human resource management subsystem(6/8)

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Ceiling on total workforce

 Workforce Level Needed level does not translate necessarily into actual hiring goal

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The human resource management subsystem(7/8)

Most new hires per FT Exp Staff Full-time-equivalent

Exp workforce

KAIST SE LAB 2010

Ceiling on new hires

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Workforce gap

 Workforce Level Sought = MIN(CELTWF, WFNEED)

• CELTWF = CEiLing on Total WorkForce

• WFNEED = WorkForce level NEEDed

 Workforce gap = Total Workforce – Workforce level sought

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The human resource management subsystem(8/8)

KAIST SE LAB 2010

Total Workforce Workforce gap > 0

Workforce gap < 0

Transfer workforce out of the project Hire new employees

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Software production subsystem

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The model’s other subsystem(1/2)

KAIST SE LAB 2010 Software developed Software tested Software development productivity Software development rate Software

testing rate Software testing productivity Man

(17)

Planning Subsystem

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The model’s other subsystem (2/2)

Indicated workforce = Man-days remaining

Time remaining

Required time to complete project

Man-days remaining Workforce Level sought

40 =

(18)

NASA‟s DE-A software development project

 NASA‟s launch of the DE-A satellite was tied to the completion of the DE-A software

 Because of strict deadline, workforce curve rises in the final stages of the project  Brooks‟ law?

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Model experimentation (1/2)

“Adding manpower to a late software project

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The simulated project behavior

 The model‟s results conformed quite accurately to the project‟s actual behavior

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(20)

 „Time Parameter‟ as a hiring policy

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Applicability of Brook’s law to the DE-A Project (1/2)

Simulating DE-A with the time parameter set to 100

(Time parameter = Hiring delay + average assimilation delay In this case, WCWF = WCWF-1)

No more hiring on the project‟s final stage

It shows that the simulated project takes 60 more days than actual project

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 Training & communication overhead vs. productivity gained

Net impact of adding more people

 The cumulative contribution

21 / 24

Applicability of Brook’s law to the DE-A Project (2/2)

Impact of different staffing policies on cost and schedule

Brook‟s law only holds in this range

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Contribution

 An integrative model that captures the multiple functions of software development

 The computer simulation tools of system dynamics

 A formal model of the software development process

Future work

 Model enhancement

• Extending on requirements analysis phase, multiple projects, large-scale project, quality

 New modeling applications

22 / 24

Conclusion

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Pros

 It gives comprehensive simulation model

Cons

 Considering this paper(book)‟s scope, more case studies are needed

23 / 24

Discussion

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(25)

Brooks‟ law

 “Adding manpower to a late software project makes it later”

» F. P. Brooks, Fr. The Mythical Man-Month.

 It has not been formally tested

25 / 24

KAIST SE LAB 2010

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Average assimilation delay

 Formulated as a first-order exponential delay

26 / 24

Average assimilation delay

LEVEL (L) Rate (R) Average Time Delay (T) At any time (t), R(t) = L(t) / T Also, d/dt L(t) = -R(t) = -L(t)/T Finally, L(t) = L(0)e-t/T

And it can be shown that the average time spent in the delay = T

(27)

Communication overhead

 Increases in proportion to n2 where n is the size of team • >> F. P. Brooks, Jr. The Mythical man-month

• >> H. D. Mills, “Software development,” IEEE Trans. Software Eng. Vol. SE-2 no. 5, Dec. 1976

27 / 24

Communication overhead

(28)

How is progress measured in a s/w project? (1/2)

 In the early phase, progress is measured by

the rate of expenditure of resources

 In this situation,

MDPRNT = MDPNNT

• MDPRNT : Man-Days Perceived Remaining for New Tasks

• MDPNNT : Man-Days Perceived still Needed for New Task

28 / 24

How is progress measured? (1/2)

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How is progress measured in a s/w project? (2/2)

MDPRNT = MDPNNT

29 / 24

How is progress measured? (2/2)

KAIST SE LAB 2010

MDPNNT = TSKPRM / PRDPRD MDPRNT = TSKPRM / PRDPRD PRDPRD = TSKPRM / MDPNNT

As a result, the assumption suggests that value of productivity is solely a f u n c t i o n o f f u t u r e p r o j e c t i o n s

TSKPRM : Tasks Perceived Remaining

PRDPRD : Perceived Development Productivity

As the project advances towards its final stages,

ACTPRD = CUMTKD / CUMDMD

ACTPRD : Actual Development Productivity CUMTKD : Cumulative Tasks Developed CUMDMD : Cumulative Man-Days Expended

(30)

De-A Project’s actual statistics

De-A Project‟s actual statistics

KAIST SE LAB 2009

Name Value

Project size 24000 DSI

Cost - Initial estimate 1100 man-days

Cost - actual 2220 man-days

Completion time - initial 320 days

Completion time - actual 380 days

Staffing-type parameter

Average productivity of exp. staff vs. that for new hires 2 : 1

Hiring delay 1.5 months

Average assimilation delay 1 month

Average transfer delay 2 weeks

Turnover rate 20 percent

References

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