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Gross Estimates

Learn the background of the Brooks Formula for software estimates and how to apply it in project management. Understand the key elements of the formula and how to calculate effort, supervision, contingency, and deployment. Find out how to optimize team size and time allocation for successful project completion using the updated formula.

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Gross Estimates

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  1. Gross Estimates The “Brooks” Formula Patrick Boens 12/05/2010

  2. Agenda • Background • Context & Objectives • Formula • Base • Updated • Developers’ focus • Supervision • Contingency • Deployment

  3. Background • Frederick Brooks is a Software Engineer and Computer Scientist, best-known for managing the development of IBM 0S/360. • Author of the famous “The Mythical Man-Month”, a book about Software Engineering and Project Management. • He had mistakenly added more workers to a project falling behind schedule, considering the equivalence of “resources” and “time” (Man-Month). • In his book, he presents a simple formula for software estimates, a formula he considered to be invalid in subsequent editions. • Still, this formula is a very good basis for early estimates.

  4. Context & Objectives • Early stages of a project (inception, initiation) • Gross != precise  it gives an idea, a rough estimate (no equivalence with COCOMO II for example)

  5. Base Formula • Effort = 1/3 specs + 1/6 code + 1/4 ut + 1/4 it … where • specs = requirements + analysis • code = build • ut = unit tests • it = integration testing

  6. Updated Formula • Some refinements are needed in order to cope with “Prince 2” sort of slicing: • Specs = Requirements + analyses (BA, FA, TA) = 1/3 = 4/12 • Code = Code + Unit tests = 1/6 + 1/4 = 5/12 • Tests = Integration Testing + Functional Testing* = 1/4 = 3/12 *UAT is not included (not a concern of IT)

  7. Updated Formula • Supervision effort (PM and Team Leader work)  not included • Contingency  not included • Deployment effort  not included • The formula becomes: • Specs = Requirements + analyses = 1/3 = 4/12 • Code = Code + Unit tests = 1/6 + 1/4 = 5/12 • Tests = Integration Testing + Functional Testing = 1/4 = 3/12 • + supervision + contingency + deployment effort

  8. Updated formula: Developers’ focus • Developers focus on: • Specs = Requirements + analyses = 1/3 = 4/12 • Code = Code + Unit tests = 1/6 + 1/4 = 5/12 • Tests = Integration Testing + Functional Testing = 1/4 = 3/12 • + supervision + contingency + deployment effort

  9. Updated formula: Supervision • Supervision is often seen as a percentage of the overall project (± 15%*) • Example: Specs + Code + Tests = 100 mdays  Supervision = 15 mdays • Total effort = 100 mdays + 15 mdays = 115 mdays * Although acceptable, a more precise formula exists to calculate the Supervision effort. we’ll see this in a minute

  10. Updated formula: Contingency • Contingency is a way to foresee what’s impredictable. Statistically, 17% contingency is used. This percentage must be adapted to the context of your project, based on the degree of uncertainty. • Example: Specs + Code + Tests + Supervision = 115 mdays  Contingency = 20 mdays • Total effort = 100 mdays + 15 mdays + 20 mdays = 135 mdays

  11. Updated formula: an example • Imagine a developer’s evaluation of 50 mdays, and now calculate the overall effort  1/12 = 10 mdays • specs = 4/12 = 40 mdays • code = 5/12 = 50 mdays • tests* = 3/12 = 30 mdays • Effort = 40 mdays + 50 mdays + 30 mdays = 120 mdays • Supervision = 120 mdays x 15/100 = 18 mdays • Contingency = 138 mdays x 17/100 = 23 mdays • Deployment = x mdays (say, 5)  Total effort = 161 mdays + 5 mdays = 166 mdays * UAT not included

  12. Updated formula: the bottom line • You started with 50 mdays and you ended up with 166 mdays! • This sounds funny: an experienced Project Manager said that he always multiplied the developers’ evaluations by PI  50 mdays x 3.141592 = 157 mdays! ... which is close enough to the end result (it remains a gross figure!) * UAT not included

  13. Supervision Effort : ≠ 15% • Time and team size must be balanced (theoretical view). A project of 6 months should be worked out with 6 people (team size and duration are equilibrated = square root). Always favor “more time” instead of “more people” (get back to Brooks’ findings). • A PM is needed for the whole duration of the project. If a month work is 20 days of work, you need the PM during 6 x 20 days, x% of his time. • The direct span of control is about 6 people*  for a project of 3 months, you need the PM half of his time; for a 6 months project, you need him full-time. • Getting back to our example of 50 mdays of developers’ estimates  we have calculated an effort of 120 mdays  120 / 20 = 6  SQRT(6) = 2.45  team of 2 people and timeline of 3 months ( 2 x 20 x 3 = 120 mdays) (ideal view)  PM is needed 33% of his time (2 persons for a possible span of 6) during 3 months  Supervision effort = 3 x 20 x 1/3 = 20 mdays (instead of 18 mdays with 15%). • Contingency of 17%  (120 + 20 ) mdays x 17/100 = 24 mdays • Deployment (5 mdays)  Total effort = 120 + 20 + 24 + 5 = 169 mdays  remember, you started from 50! * Software industry

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