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Software Cost Estimation Basics: COCOMO & Function Points

Learn the basics of software cost estimation using COCOMO and Function Points. Understand the principles of decomposition and empirical knowledge. Estimate projects accurately using parameters like lines of code, complexity, and skills involved.

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Software Cost Estimation Basics: COCOMO & Function Points

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  1. COST ESTIMATION Basics, COCOMO, FP

  2. What is estimated? • TIME <=> MONEY • TIME: • duration, chronological weeks, months, years • effort, person-month (man-month) • unit is typically month (year)

  3. Basic Relation • person-month / nbr-of-people = duration • example: • 36 person-month, 1 developer, 36 month • 36 person-month, 2 people, 18 month • 36 person-month, 12 people, 3 month • Beware of the “million monkey” syndrome [Brooks]

  4. Basic Principles - Decomposition • It is easier to estimate time & cost for a smaller unit than to estimate time & cost for the complete system. • Estimation by decomposition: • Decompose your system or development process, estimate time & cost for the parts and summarize.

  5. Basic Principles - Empirical Knowledge • Find parameters which characterise your system and derive time & cost based on a documented case history. • Empirical model: • time = function (parameters) • You rely on a correlation between time & cost and the set of independent parameters

  6. Typical Parameters • number and complexity of functions • number and complexity of modules • number of lines of code (LOCS) • number and duration of steps in your development process • skills and technologies involved

  7. When to do your estimate • the later the more accurate your parameters • early estimate: • you need to estimate your parameters • divide an conquer strategy • garbage in => garbage out • keep your estimate up-to-date

  8. COCOMO [B. Boehm] • COnstructive COst MOdel • empirical method • hierarchy of COCOMO models (parameters) • basic: prg size • intermediate: prg size & cost drivers • advanced: prg size & cost drivers per phaseCost Drivers: Personnel/Team Capability, Product Complexity, Req. Reliability, Problem Domain Experience, Development practices, Tools, Language Experience, etc. • for each project type

  9. COCOMO - Parameters • prg size: LOCS (lines of code) • “cost drivers” • evaluate: • product, hardware, personnel, project • measured on a 6 point scale • allow for derivation of an EAF • (Effort Adjustment Factor)

  10. COCOMO - Project Types • organic mode: • relatively small and simple • small team of skilled developers • semi-detached: • intermediate in size and complexity • mixed skill levels on the team • embedded: • hard constraints for HW, SW, operation

  11. Basic COCOMO - The Functions person-month = a*(KLOC)b duration-in-month = c*(person-month)d KLOC ... kilo lines of code a, b, c, d depend on the project type

  12. Basic COCOMO - Tables project type: a b c d organic 2.4 1.05 2.5 0.38 semi-detached 3.0 1.12 2.5 0.35 embedded 3.6 1.20 2.5 0.32

  13. Basic Cocomo - Example • You must decide on: • LOCS: 50,000 lines of code • project type: semi-detached • person-month: 3.0*(50)1.12 ~ 240 • duration-in-month: 2.5*(240)0.35 ~ 17 • number of people: 240 / 17 ~ 14

  14. Function Points (FP) • Based on the number and complexity of the system functions to be delivered to the customer • Steps: • (1) Categorize the functions according to type (input, output, database, interface, etc.) and complexity (simple, moderate, average, complex, highly complex) • (2) Derive the number of function points: multiply the number of functions in each category by the appropriate complexity weights and total the number of PF • (3) Determine the total Project Influence Factors (PIF): PIF types (distributed processing,multiple development sites, etc.), and levels of difficulty ( from 0-no difficulty, to 3-average difficulty, to 5- great difficulty) • (4) Compute the Total Effort: PM = .036 * FP * PIF

  15. Function Diff. Level Number Complexity FP M 2 Input 3 6 A 4 DB 10 40 Output S 4 3 12 58 Total PIF Total FP: Example All values from FP tables. (1&2) PF (3) PIF (4) Total Effort Difficulty Level 2 Data performance objectives Multiple sites 5 7 PM = 0.036*58*7 = 14.616

  16. Function Points: Evaluation • Tightly coupled with Functional Requirements (see SRS) - works well with DFDs • Intuitive Method - makes intuitive sense to the customers • Notice: A rough estimate of the functional requirements will yield ONLY a very rough estimate of total effort

  17. “3 Time Programming” Rule • Estimate the number of software modules/programs and their complexity • Estimate the effort required to complete coding and debugging of each module and the total programming effort • Total Project Effort = (Estimated Programming Effort) * 3 This includes Management, Documentation, Design, …. => Helpful ONLY to experienced programmers => Corrects at least the OPTIMISTS A “Rough Guess” is better than a “Wild Guess” or “No Estimate”

  18. It Couldn’t be More Simple: Just Try Doing It!

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