Effort metrics: Man-month Mythical Man Month – the book



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Effort metrics: Man-month

Mythical Man Month – the book

  • Brooks lead development of OS/360 and reflected on the problems experienced in the project.
  • The book consists of a number of essays.
  • The1986-paper started a lot of debate.
  • Countless reviews, discussion, etc. on www.
  • The book is widely recognized and – to a certain extent – timeless. Source of contemplation.
  • ”Large-system programming has […] been a tar pit […]. Few have met goals, schedules, and budgets. […] Team after team has become entangled in the tar. No one thing seems to cause the difficulty[…]. But the accumulation of simultaneous and interacting factors brings slower and slower motion. Everyone seems to have been surprised by the stickiness of the problem, and it is hard to discern the nature of it. But we must try to understand it if we are to solve it.”
  • Mythical Man Month 1st ed. Ch.1-15 (1975)
  • Reflection on NSB Ch.17 (1995)
  • Reflection on MM-M Ch.18-19 (1995)
  • No Silver Bullet IFIPS paper Ch.16 (1986)

Essay: The Mythical Man Month

  • More software projects have gone awry due to lack of calendar time than for all other causes combined. Why is this cause of disaster so common?
    • Poor estimation techniques.
    • Gutless estimating.
    • Poor progress monitoring.
    • Schedule slippage ”solved” by adding manpower.
    • Confuse effort with progress.

Man- Month

  • The man-month as a unit for measuring the size of a job is a dangerous and deceptive myth.
  • It implies that men and months are interchangeable.

Il mese-uomo (1)

Men/Months interchangable

  • Men and months are interchangeable commodities only when a task can be partitioned among many workers with no communication among them (Fig. 2.1). This is true of reaping wheat or picking cotton; it is not even approximately true of systems programming!

Men/Months interchangable

  • The bearing of a child takes nine months, no matter how many women are assigned!

Men/Months interchangable

  • In tasks that can be partitioned but which require communication among the subtasks, the effort of communication must be added to the amount of work to be done

Men/Months interchangable

  • If each part of the task must be separately coordinated with each other part/ the effort increases as n(n-I)/2. The added effort of communicating may fully counteract the division of the original task

Il mese-uomo (2)

  • Andamento reale della curva tempo-risorse

Communication Overhead

  • 5 Members
  • 10 Interactions

Peculiarità del software

    • Complessità
    • Labilità
    • Modificabilità
    • Invisibilità

Men/Months interchangable

  • The plan
  • First task, twice as long

Men/Months interchangable - Lessons

  • Brooks’ law:
    • Adding man power to a late project makes it later.
    • Calculate overhead when adding man power.
    • Use quick starters.
    • Take competences into account.
  • Today:
    • Plan the project so that slippage are revealed earlier (iterations + use cases).

People and Effort

  • “If we fall behind schedule we can always add more programmers and catch to late in the project”
  • Has a disruptive effect on the project

People and Effort

  • The relationship between the number of people working in software project and overall productivity is
  • not linear

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