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Software Estimation: What, Why & How. Nupul Kukreja 9 th October 2013. Based On Software Estimation: Demystifying The Black Art. Steve McConnell Microsoft Press. Agenda. What is an “Estimate”? Purpose of Estimation In-class quiz to gauge & develop your estimation skills

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software estimation what why how

Software Estimation: What, Why & How

Nupul Kukreja

9thOctober 2013

  • What is an “Estimate”?
  • Purpose of Estimation
  • In-class quiz to gauge & develop your estimation skills
  • What are “Good Estimates”?
  • Estimation and Cone of Uncertainty
  • Estimation Techniques
agenda vbse 4 1 view
Agenda: VBSE 4+1 View

5a, 7b. Options, solution development & analysis

Dependency Theory

Utility Theory

3. SCS Value Propositions (Win Conditions)

2a. Results chains

2. Identify SCS

3b, 5a, 7b. Cost/schedule/ performance tradeoffs

4. SCS expectations management

Theory-W:SCS Win-Win

3b, 7a. Solution Analysis

5a, 7b. Prototyping

5. SCS WinWin Negotiation

6, 7c. Refine, execute, monitor & control plans

1. Protagonist goals

3a. Solution Exploration



7. Risk, opportunity, change management

5a. Investment analysis, Risk analysis

6a, 7c. State measurement, prediction correction; Milestone synchronization

what is a n estimate
What Is An ‘Estimate’?
  • Estimate: Prediction of duration or cost of project
  • Target: Statement of desirable business objective Ex.:
    • We need to have product ready by Christmas
    • We must limit cost of next release to $3 million owing to budget constraints.
  • Commitment: Promise to deliver defined functionality at specific level of quality by defined date
  • Commitment != Estimate (doesn’t have to be )
why do we estimate
Why Do We Estimate?
  • To determine if project’s targets are realistic enough to control progress to meet them
  • And NOT to predict a project’s outcome
  • Estimate vs. Target Gap:
    • ≤ 20%  Easy to control feature set, schedule, team size towards realization
    • Else  Not possible to control project towards successful realization  Targets need to be better aligned with reality 
estimates vs plans
Estimates vs. Plans
  • Estimates form foundations for plans
  • Plans don’t have to be same as estimates
    • If (Targets – Estimates) >> 1
      • Plans must account for high risk
    • Else
      • Plans can assume less risk
  • Planning considerations that partially depend on accurate estimates:
    • Creating a detailed schedule
    • Prioritizing functionality for delivery
    • Breaking project into iterations etc.
estimates as probability statements
Estimates As Probability Statements
  • Single-point estimates “assume” 100% odds of success; NOT realistic
  • Usually a ‘Target’ in disguise 
  • Must factor in uncertainty  i.e. project success follows a probability distribution


Nominal Outcome

Nominal Outcome


Schedule (or Cost of Effort)

Schedule (or Cost of Effort)

Common Assumption

More Realistic

what is a good estimate
What Is A Good Estimate?

“An estimate that provides a clear enough view of the project reality to allow the project leadership to make good decision about how to control the project to hit its targets”

As quoted from Steve McConnell’s book.

how good an estimator are you
How Good an Estimator Are YOU?
  • Estimate the questions (in the following slide) to the best of your ability
  • Take a few WAGs if you will 
  • Fill lower/upper bound so that there is a 90% chance of including the correct value
  • This is a quiz on estimation and NOT on Googling skills. Estimate without any electronic, human or supernatural help 
love to gamble
Love to Gamble?

Let’s say we gave you the following proposition:

  • You win $1000 if the actual answer is within your range
  • You win $1000 by spinning the wheel below:
  • What would you choose?

Your estimate/range or spin-the-wheel?




Win $1000

implications of choice
Implications of Choice


Set range just right so as to be indifferent between gamble and your estimate i.e. 90% chance, not more and not less, that answer is within range

90% Confident  90% of the time 90% of the answers within range (i.e. you get 9 answers correct on 9 of 10 such quizzes)

accuracy of estimates
Accuracy of Estimates
  • Is it better to overestimate or underestimate?
accuracy of estimates1
Accuracy of Estimates

Effort, Cost, Schedule

Nonlinear penalty due to planning errors, upstream defects, high-risk practices

Linear penalty due to Parkinson’s Law



< 100%


> 100%

Target as a Percentage of Nominal Estimate

Penalties of underestimation more severe than those for overestimation. If you can’t estimate with complete accuracy, it’s better to err on the side of overestimation

– Steve McConnell

cone of uncertainty
Cone of Uncertainty

16x Error range!










Initial Concept

Approved Product Definition

Requirements Complete

User Interface Design Complete

Detailed Design Complete

Software Complete

The more refined the software’s definition the more accurate the estimate

front loaded cone of uncertainty
“Front-Loaded” Cone of Uncertainty

The cone narrows itself only if you make decisions that eliminate variability. Else it’d just blow up even more






Software Complete


Detailed Design Complete


User Interface Design Complete


Requirements Complete


Approved Product Definition

Initial Concept

Milestones usually ‘front-loaded’  Improved estimation accuracy for first 30% of project i.e. from ±4x to ±1.25x

using the cone of uncertainty
Using the Cone of Uncertainty
  • If using a single point estimate:
    • Come up with estimate
    • Use multiplying factors from previous chart for relevant milestone to get range
count compute judge
Count, Compute, Judge
  • Count First – If you can count something directly please do so 
  • If you can’t count the answer directly, count something else (i.e. correlated to the item you wish to estimate) and compute the answer (preferably by using calibration data)
  • Use judgment as a last resort
fermi lize your estimation skills
Fermi-lize Your Estimation Skills
  • Enrico Fermi – Won a Nobel Prize in Physics in 1938. Well known for his creative and intuitive, even casual sounding estimates
  • A "Fermi question" is a question which seeks a fast, rough estimate of quantity which is either difficult or impossible to measure directly
1 fermi decomposition
1. Fermi Decomposition
  • Figure out something that is known about the quantity in question
  • Estimate other things that may have a bearing on that quantity – it’s okay to have rough approximations
  • Sometimes you can just Google some numbers to extrapolate from there!
  • It’s ALWAYS possible to estimate and get ballpark idea about the quantity in question 
team in class
Team In-Class
  • How many golf balls can fit in this class room (OHE 123)?*

*DEN Students: You may think of the room you are viewing the lecture in 

2 individual expert judgment
2. Individual Expert Judgment
  • Most common estimation approach
  • Experts = those are doing the task 
  • These are ‘task-level’ estimates i.e. specific tasks like feature development, testing etc.,
  • Task level estimation: Decompose estimates into tasks requiring no more than 2 days of effort (rule of thumb to avoid estimation error)
individual expert judgment
Individual Expert Judgment

Example of developer single-point estimates (not preferable)

Example of individual estimation using best case and worst case. Provides better estimates and forces thinking of worst case estimates – leading to better overall range

individual expert judgment1
Individual Expert Judgment

Even Better: Compute a 3-point estimate including the most-likely case

Compute expected case using the PERT formula:

Expected Case = [BestCase + (4* MostLikelyCase) + WorstCase]/6

individual expert judgment2
Individual Expert Judgment

Compare estimates to actuals to improve estimation accuracy over time and ‘narrow’ the cone of uncertainty

Magnitude of Relative Error: |(Actual – Estimate)/Actual|

3 decomposition recomposition
3. Decomposition & Recomposition
  • Process:
    • Separate an estimate into multiple pieces
    • Estimate each piece individually
    • Recombine individual estimates into an overall aggregate estimate
  • AKA “bottom up” estimation or “Work Breakdown Structure (WBS)
  • Very important and highly used technique
  • Leads to quite accurate estimates
work breakdown structure wbs
Work Breakdown Structure (WBS)
  • Example: Cost of owing and operating a car
    • Buy the car
      • Pay down payment
      • Pay taxes, licensing and registration fees
      • Insurance the car
      • Pay monthly loan installments
    • Operate and maintain the car
      • Pay semi-annual insurance payments
      • Fill car with gas when needed
      • Change oil every 3000 miles
        • Take car to oil change shop
        • Let them do work
        • Pay fees and taxes
        • Drive back
      • Other routine maintenance
    • Sell the car

Estimate each piece individually and aggregate the estimates all the way to the top

Law of Large Numbers:

Overestimating some pieces will help cancel out some of the underestimates of the rest. Leading to better estimates

4 estimation by analogy
4. Estimation by Analogy
  • Create estimates of new project by comparing it to a similar past project
  • Can help create accurate estimates (by following the process below, instead of relying on memory)
    • Get detailed size, effort and cost results for similar previous project (WBS is preferable if possible)
    • Compare size of new, piece-by-piece to previous
    • Build up estimate for new project’s size as a percentage of old project’s size
    • Create an effort estimate based on size of new project compared to that of previous one
    • Check for consistent assumptions across the two
5 proxy based estimates
5. Proxy-Based Estimates
  • Very difficult to estimate SLOC count looking at a feature
  • Or expected #defects, #test-cases, #classes etc.,
  • Proxy-based estimation:
    • Identify a proxy that is correlated with quantity to be estimated
    • Proxy is usually easier to estimate/count or available sooner in project
    • Compute estimate based on proxy and past historical data
  • Useful for creating whole-project or whole-iteration estimates but NOT for detailed task-by-task or feature-by-feature
  • Example of two proxy-based estimates…
story points
Story Points
  • Unit-less measure of ‘complexity’ or ‘size’ of feature
  • Scales:
    • Powers of 2: 1, 2, 4, 8, 16 …
    • Fibonacci: 1, 2, 3, 5, 8, 13 …
  • #Story-Points per iteration = Velocity i.e. looking at past velocity estimate completion time of project (use historical data if available or forecast or run one iteration)
t shirt sizing
T-Shirt Sizing
  • Break features into various (albeit fuzzy) T-shirt sizes
    • Small, Medium, Large, X-Large, XX-Large etc.
  • Estimate Business Value and Ease of Realization using T-shirt sizes for each feature
  • Gauge overall feature value based on responses
  • Provides crude, sufficiently accurate, initial high-level estimates in the wide part of the cone
t shirt sizing score chart
T-Shirt Sizing Score Chart
  • Can create one that suits the organization or use the one below as per Steve McConnell:
  • Use scores from above chart to compute approximate net business value for each feature (in sorted order)
6 expert judgments in groups
6. Expert Judgments in Groups
  • Similar to planning poker 
  • Have each team member estimate pieces of project individually
  • Meet to compare the estimates
  • Reach mutual consensus as group
    • Without averaging the estimates. You may average the estimates but you still need to discuss individual results
  • Extremely effective technique to help improve estimation accuracy
7 estimation by tools
7. Estimation by Tools
  • Helps perform tasks that can’t be done manually
    • Simulating project outcomes (i.e. sensitivity analysis)
    • Probability Analysis i.e. viewing the (cumulative) probability distribution of the estimates
    • What-if analyses
    • Serves as referee for unrealistic project expectations
    • Estimation of less common software issues
  • Works best if you have historical data for calibration
list of available tools
List of Available Tools
  • Construx Estimate
  • Costar (commercial implementation of COCOMO II)
  • Price-S
  • SEER
  • SLIM-Estimate and Estimate Express
  • Your own home grown one?
using multiple approaches
Using Multiple Approaches
  • No single technique is perfect
  • Best to augment estimation with multiple approaches
  • Each approach works best in specific context
  • Convergence amongst estimates suggests a good estimate
  • Divergence helps see if something was overlooked or needs to be understood better
  • Estimation is NOT an art (somewhat but not entirely)
  • Can effectively be executed as a science
  • Need not rely purely on intuition or memory
  • Improves over time especially if historical data is captured
  • Requires basic arithmetic to understand
  • Complex models can be created with the help of statistics – premise of most ‘tools’
  • VERYYYYYYYYYYYYYYYY Critical skill-set to have in the 21st Century 