MTAT.03.244 Software Economics
Software Cost Estimation
Marlon Dumas
marlon.dumas ät ut . ee
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What do we have until now • Function points: a relatively objective (though
not fail-proof) way of estimating software size • We can use this to:
• Compare software systems in terms of size • Track achieved progress in a software project
• So if there are disputes, there is an “objective” way to determine how much to pay for delivered functionality
• But we don’t know yet anything about costs…
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For Discussion It is hopeless to accurately estimate software
costs. Most often than not, such estimates are wrong. So why should we bother?
We have 6 months and 10 analysts/developers, so it will take 6 months and 60 person-months. Why bother about estimating the cost?
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Two lessons • You can be uncertain of something, but you first
need to have something to be uncertain of • You can change your plan, but you first need to
have something to change
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What is a Good Estimate? “A good estimate is an estimate that provides a
clear enough view of the project reality to allow the project leadership to make good decisions about how to control the project to hit its targets.”
Steve McConnel Software Estimation: Demystifying the Black Art
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Good (ARTfUL) Estimates • Accurate (reasonably) • Reliable (reasonably) • Traceable: we should know where the effort will
go into and why? • Updatable: it should be easy to “refine” with
new data
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There are lies, dammed lies and statistics.
• What about a method to estimate software costs from a high-level architecture, that is: • within 20% of the actual size 50% of the time • within 30% of the actual size 66% of the time • Decomposes effort/cost into four project
phases • Can we use it and how?
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Cone of Uncertainty
Craig Larman Agile & Iterative Development
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Effort Estimation Parkinson's Law?
If we have 5 person-years, it will take 5 person-years
Estimation by analogy (nearest neighbour) This project is 20% more complex than the previous one
Expert judgement (multi-source estimation) Wideband Delphi Planning Poker
Parametric (statistical) methods
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Wide-Band Delphi Ask each team member their estimate
Apply personal experience, Look at completed projects, Extrapolate from modules known to date
Collect and share in a meeting: discuss why/how different people made their estimate
Repeat When stable, Size = (H + 4 X Ave. + L)/6
See: http://www.stellman-greene.com/ch03
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Fun Variant: Planning Poker 1. Product owner reads a user story, answers
questions 2. Simultaneously: each team member pulls a
card with their estimate (e.g. in “story points”, “ideal” days, etc.)
3. The holders of smallest and the largest estimate give reasons (others discuss too)
4. Repeat from 2 until convergence 12
Statistical Methods • Evidence-based scheduling:
www.joelonsoftware.com/items/2007/10/26.html
• Fitting a function-points-to-effort function using history www.math.vu.nl/~x/ipm/ipm.pdf
• Parametric cost models • COCOMO II.2000 (Boehm et al.) • Costar and Cost Xpert (based on COCOMO II) • SLIM-Estimate • Construx Estimate, KnowledgePlan, TruePlanning, ...
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Six forms of software cost estimation (Caper Jones)
Project
Phase
Activity
Project
Phase
Activity 14
Principle of Parametric Estimation It took me one month to fully develop (end-to-end)
a small software application of 1000 LOC Can I develop an application of 10000 LOC in 10
months? I have four friends with similar experience as
mine, can we develop an application of 10000 LOC in 2 months?
Hints: Brook’s law, Farr & Nanus study
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Non-Linear Productivity There is overwhelming evidence that, except for
simple projects, development effort goes up exponentially with size, so this is probably wrong: Effort = P x Size
This might be closer to the mark: Effort = A x M x SizeB
where A is a constant derived from historical data, and M is dependent on each project (effort multiplier), and B is dependent on the complexity of the project
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Approach 1 - Discretize
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Proceedings 5th Software Measurement European Forum, Milan 2008
129
It is evident that the “base productivity” can be often a value not very significant to
determine the final effort, but in any case you have to start by an initial value. The first effort driver in order to calibrate the “base productivity” is the functional dimension of the projects. Table 1 shows the “base productivity” for Java related to the functional dimension in FP: with the growth of dimension, the productivity decreases by two points; there are five classes of projects. The productivity is expressed in fp/person month.
Table 1
Applying these values to the dimension in FP of a project we obtain a base work effort of
the software development project in Month/person or in Days/person (usually we consider a month of 21 days).
This effort should cover all the activities concerning the software project according to the
RUP (Rational Unified Process) disciplines. For example if there is a project valued 574 FP, from the table above we obtain a “base
productivity” of 16 fp/month and thus a “base work effort” of 35.87 Month/person(574/16) or 753 days/person.
In the case of EMP it is to consider not only the functional dimension of EMP (in Nesma
FP) but also the functional dimension of the application on which the maintenance will be applied to for selecting the base productivity, (for example if we had an EMP for our projects of 574 FP, we will choose a productivity of 16 fp/month, independently of the EMP Nesma functional dimension in FP).
4. The “Productivity Factors”
Every project has its own characteristics, so it is clear that the range of productivity can be
very large! The most critical step in evaluating the effort of the project is to consider all the factors that can influence the productivity. Through the Functional Requirements it is possible to measure the functional dimension in Function Points.
The Technical and Quality requirements should give us information about the factors that influence productivity. But how can we measure them? And, above all, which are these factors?
In 2003 in CSI Piemonte was done an investigation to find them and to evaluate the impact that they have on productivity (growing or decreasing it). To determine the impact of productivity factors it was chosen an approach like COCOMO [6] (the productivity factor is the multiplication of several cost drivers).
Cf. • ISBSG.org • Capers Jones database
Source: Function Point: how to transform them in effort? by Gianfranco Lanza
Quick Exercise • An application for managing construction
equipment rentals has an estimated size of 400 FPs
• How much will it cost?
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Approach 2 – Interpolate Nonlinear relationship when exponent > 1
0
2 0 0 0
4 0 0 0
6 0 0 0
8 0 0 0
1 0 0 0 0
1 2 0 0 0
1 4 0 0 0
1 6 0 0 0
0 5 0 0 1 0 0 0
K S L O C
Pe
rso
n M
on
ths B = 1 .2 2 6
B = 1 .0 0
B = 0 .9 1
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COCOMO - Constructive Cost Model
• Developed at USC (Barry Boehm et al.) based on a database of 63-161 projects
• First version of COCOMO (now COCOMO 81) Most recent version COCOMO II.2000
• Based on statistical model building (fitting actual data to equation)
• Not very accurate with default database - should be calibrated based on company-specific historical data
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COCOMO II Designed for an iterative development method
(MBASE) More refined set of cost drivers (6-17) Multiple exponential scale drivers:
PM = a x Sizeb x Π EMi (i = 1 to 6 or 17) where a = 2.94
b = 0.91 + 0.01 x Σ SFj (j = 1 to 5)
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COCOMO II models COCOMO II incorporates a range of sub-models that produce
increasingly detailed software estimates. Sub-models in COCOMO II:
Application composition model. Used when software is composed from existing parts.
Early design model. Used when requirements are available but design has not yet started (6 cost drivers).
Reuse model. Used to compute the effort of integrating reusable components.
Post-architecture model. Used once the system architecture has been designed and more information about the system is available (17 cost drivers).
From I. Sommerville’s Software Engineering 22
Use of COCOMO II models
From I. Sommerville’s Software Engineering 23
Cost Factors Significant factors of development cost:
scale drivers are sources of exponential effort variation
cost drivers are sources of linear effort variation product, platform, personnel and project attributes effort multipliers associated with cost driver ratings
Defined to be as objective as possible Each factor is rated between very low and very
high per rating guidelines relevant effort multipliers adjust the cost up or down
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Scale Drivers Precedentedness (PREC)
Degree to which system is new/past experience applies
Development Flexibility (FLEX) Need to conform with specified requirements
Architecture/Risk Resolution (RESL) Degree of design thoroughness and risk elimination
Team Cohesion (TEAM) Need to synchronize stakeholders and minimize conflict
Process Maturity (PMAT) SEI CMM process maturity rating 25
Scale Factors Sum scale factors SFi across all of the factors
to determine a scale exponent, B, using B = .91 + .01 Σ SFi
Scale Factors (Wi) Very Low Low Nominal High Very High Extra High
Precedentedness(PREC)
thoroughlyunprecedented
largelyunprecedented
somewhatunprecedented
generallyfamiliar
largelyfamiliar
throughlyfamiliar
DevelopmentFlexibility (FLEX)
rigorous occasionalrelaxation
somerelaxation
generalconformity
someconformity
generalgoals
Architecture/RiskResolution (RESL)*
little (20%) some (40%) often (60%) generally(75%)
mostly(90%)
full (100%)
Team Cohesion(TEAM)
very difficultinteractions
some difficultinteractions
basicallycooperativeinteractions
largelycooperative
highlycooperative
seamlessinteractions
Process Maturity(PMAT)
Weighted average of “Yes” answers to CMM Maturity Questionnaire
* % significant module interfaces specified, % significant risks eliminated
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Precedentedness (PREC) and Development Flexibility (FLEX)
Feature Very Low Nominal / High Extra High
PrecedentednessOrganizational understanding of productobjectives
General Considerable Thorough
Experience in working with related softwaresystems
Moderate Considerable Extensive
Concurrent development of associated newhardware and operational procedures
Extensive Moderate Some
Need for innovative data processingarchitectures, algorithms
Considerable Some Minimal
Development FlexibilityNeed for software conformance with pre-established requirements
Full Considerable Basic
Need for software conformance withexternal interface specifications
Full Considerable Basic
Premium on early completion High Medium Low
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Architecture / Risk Resolution (RESL) Use a subjective weighted average of:
Characteristic Very Low Low Nominal High Very High Extra High
Risk Management Plan identifies all criticalrisk items, establishes milestones forresolving them by PDR.
None Little Some Generally Mostly Fully
Schedule, budget, and internal milestonesthrough PDR compatible with RiskManagement Plan
None Little Some Generally Mostly Fully
Percent of development schedule devotedto establishing architecture, given generalproduct objectives
5 10 17 25 33 40
Percent of required top software architectsavailable to project
20 40 60 80 100 120
Tool support available for resolving riskitems, developing and verifyingarchitectural specs
None Little Some Good Strong Full
Level of uncertainty in Key architecturedrivers: mission, user interface, COTS,hardware, technology, performance.
Extreme Significant Considerable Some Little Very Little
Number and criticality of risk items > 10Critical
5-10Critical
2-4Critical
1Critical
> 5 Non-Critical
< 5 Non-Critical
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Team Cohesion (TEAM) Use a subjective weighted average of the
characteristics to account for project turbulence and entropy due to difficulties in synchronizing the project's stakeholders.
Stakeholders include users, customers, developers, maintainers, interfacers, and others
Characteristic Very Low Low Nominal High Very High Extra High
Consistency of stakeholderobjectives and cultures
Little Some Basic Considerable Strong Full
Ability, willingness of stakeholders toaccommodate other stakeholders'objectives
Little Some Basic Considerable Strong Full
Experience of stakeholders inoperating as a team
None Little Little Basic Considerable Extensive
Stakeholder teambuilding to achieveshared vision and commitments
None Little Little Basic Considerable Extensive
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Process Maturity (PMAT) Two methods based on the Software Engineering
Institute's Capability Maturity Model (CMM) Method 1:
Overall Maturity Level (CMM Level 1 through 5)
Method 2: Key Process Areas (see next slide)
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Key Process Areas Decide the percentage of compliance for each of
the KPAs as determined by a judgment-based averaging across the goals for all 18 Key Process Areas.
Key Process Areas Almost Always(>90%)
Frequently(60-90%)
About Half(40-60%)
Occasionally(10-40%)
Rarely If Ever(<10%)
Does NotApply
Don'tKnow
1 RequirementsManagement
r r r r r r r
2 Software ProjectPlanning
r r r r r r r
3 Software ProjectTracking and Oversight
r r r r r r r
4 Software SubcontractManagement
r r r r r r r
(See COCOMO II Model Definition Manual for remaining details)
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A company takes on a project in a new domain. The client has not defined the process to be used and has not allowed time for risk analysis. The company has a CMM level 2 rating. Precedenteness - new project – 0.4 Development flexibility - no client involvement - Very high – 0.1 Architecture/risk resolution - No risk analysis - V. Low – 0.5 Team cohesion - new team – nominal – 0.3 Process maturity - some control – nominal – 0.3
Scale factor = 1.17.
Example of Scale Factors
From I. Sommerville’s Software Engineering 32
Cost Drivers (Post-Architectural Model) Product Factors
Reliability (RELY) Data (DATA) Complexity (CPLX) Reusability (RUSE) Documentation (DOCU)
Platform Factors Time constraint (TIME) Storage constraint (STOR) Platform volatility (PVOL)
Personnel factors Analyst capability (ACAP) Program capability (PCAP) Applications experience (APEX) Platform experience (PLEX) Language and tool experience
(LTEX) Personnel continuity (PCON)
Project Factors Software tools (TOOL) Multisite development (SITE) Required schedule (SCED)
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Example Cost Driver - Required Software Reliability (RELY)
Measures the extent to which the software must perform its intended function over a period of time.
Ask: what is the effect of a software failure? Very Low Low Nominal High Very High Extra High
RELY slightinconvenience
low, easilyrecoverablelosses
moderate,easilyrecoverablelosses
high financialloss
risk to humanlife
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Example Effort Multiplier Values for RELY
Very Low Low High Very High
Slight Inconvenience
Low, Easily Recoverable
Losses
High Financial Loss Risk to Human
Life
1.15
0.75
0.88
1.39
1.0 Moderate, Easily
Recoverable Losses
Nominal
E.g. a highly reliable system costs 39% more than a nominally reliable system 1.39/1.0=1.39)
or a highly reliable system costs 85% more than a very low reliability system (1.39/.75=1.85)
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COCOMO II – Schedule Estimation
D = c x Ed x SCED%/100
where c = 3.67
d = 0.33 + 0.2 x [b - 1.01]
SCED% = percentage of required schedule compression
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Nominal versus Optimal Time
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Cocomo II Exercise • See separate handout • Use the following resources:
• COCOMO II Data Sheet • Model Definition Manual • Online cost Cocomo II calculator (see list of
Cocomo Resources under the course’s “Readings” page)
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Software Cost vs Price Caution: All of the above is about effort and
schedule estimation From effort and schedule, one can estimate cost
Estimate technical effort cost based on PM x monthy total salary cost
Add licensing costs and overhead cost for administrative support, infrastructure, etc.
But cost ≠ price Price depends on many other factors:
Risk margin, requirements volatility, competitive advantage, market opportunity, need to win a bid… 39
Final Word of Caution COCOMO and similar models are just MODELS COCOMO comes calibrated by a set of projects
that might not reflect a particular project’s context
Should be combined with expert assessment – for example, combine Cocomo with estimates based on the Work Breakdown Structures
Cost estimation should be followed by continuous cost control (more on this next week)
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Re: Homework 1 Effort and schedule estimation for your system can be done using one of the following methods: • Using Cocomo II (post-architectural).
• Explain your choice of cost and scale drivers • Add a comment after your estimate to discuss how
credible you find the estimate given by Cocomo
• Wideband Delphi using the method in http://www.stellman-greene.com/ch03
• Or planning poker over each individual feature • Provide table of feature and estimate in person-days
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