The Mythical Man-Month: Essays on Software Engineering by Frederick P. Brooks

Summary

  1. This book is a belated answer to Tom Watson’s probing questions as to why programming is hard to manage…Briefly, I believe that large programming projects suffer management problems different in kind from small ones, due to division of labor. I believe the critical need to be the preservation of the conceptual integrity of the product itself. These chapters explore both the difficulties of achieving this unity and methods for doing so. The later chapters explore other aspects of software engineering management….The Mythical Man-Month is only incidentally about software but primarily about how people in teams make things. There is surely some truth in this

Key Takeaways

  1. A rule of thumb, I estimate that a programming product costs at least three times as much as a debugged program with the same function…Programming system component costs at least three times as much as a stand-alone program of the same function.
  2. First, one must perform perfectly. The computer resembles the magic of legend in this respect, too. If one character, one pause, of the incantation is not strictly in proper form, the magic doesn’t work. Human beings are not accustomed to being perfect, and few areas of human activity demand it. Adjusting to the requirement for perfection is, I think, the most difficult part of learning to program. Next, other people set one’s objectives, provide one’s resources, and furnish one’s information. One rarely controls the circumstances of his work, or even its goal. In management terms, one’s authority is not sufficient for his responsibility. It seems that in all fields, however, the jobs where things get done never have formal authority commensurate with responsibility. In practice, actual (as opposed to formal) authority is acquired from the very momentum of accomplishment. The dependence upon others has a particular case that is especially painful for the system programmer. He depends upon other people’s programs. These are often maldesigned, poorly implemented, incompletely delivered (no source code or test cases), and poorly documented. So he must spend hours studying and fixing things that in an ideal world would be complete, available, and usable. The next woe is that designing grand concepts is fun; finding nitty little bugs is just work. With any creative activity come dreary hours of tedious, painstaking labor, and programming is no exception.
  3. The challenge and the mission are to find real solutions to real problems on actual schedules with available resources.
  4. More software projects have gone awry for lack of calendar time than for all other causes combined. Why is this cause of disaster so common? First, our techniques of estimating are poorly developed. More seriously, they reflect an unvoiced assumption which is quite untrue, i.e., that all will go well. Second, our estimating techniques fallaciously confuse effort with progress, hiding the assumption that men and months are interchangeable.
    1. Key point – men and months are not interchangeable, but we make assumptions that they are
  5. Fifth, when schedule slippage is recognized, the natural (and traditional) response is to add manpower. Like dousing a fire with gasoline, this makes matters worse, much worse. More fire requires more gasoline, and thus begins a regenerative cycle which ends in disaster.
  6. For the human makers of things, the incompletenesses and inconsistencies of our ideas become clear only during implementation. Thus it is that writing, experimentation, “working out” are essential disciplines for the theoretician.
  7. The second fallacious thought mode is expressed in the very unit of effort used in estimating and scheduling: the man-month. Cost does indeed vary as the product of the number of men and the number of months. Progress does not. Hence 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.
  8. The bearing of a child takes nine months, no matter how many women are assigned. Many software tasks have this characteristic because of the sequential nature of debugging.
  9. Since software construction is inherently a systems effort—an exercise in complex interrelationships—communication effort is great, and it quickly dominates the decrease in individual task time brought about by partitioning. Adding more men then lengthens, not shortens, the schedule. 
  10. For some years I have been successfully using the following rule of thumb for scheduling a software task: 1/3 planning 1/6 coding 1/4 component test and early system test 1/4 system test, all components in hand.
  11. Programming managers have long recognized wide productivity variations between good programmers and poor ones. But the actual measured magnitudes have astounded all of us. In one of their studies, Sackman, Erikson, and Grant were measuring performances of a group of experienced programmers. Within just this group the ratios between best and worst performances averaged about 10:1 on productivity measurements and an amazing 5:1 on program speed and space measurements! In short the $20,000/year programmer may well be 10 times as productive as the $10,000/year one. The converse may be true, too. The data showed no correlation whatsoever between experience and performance. (I doubt if that is universally true.)
  12. I have earlier argued that the sheer number of minds to be coordinated affects the cost of the effort, for a major part of the cost is communication and correcting the ill effects of miscommunication (system debugging). This, too, suggests that one wants the system to be built by as few minds as possible.
  13. The dilemma is a cruel one. For efficiency and conceptual integrity, one prefers a few good minds doing design and construction. Yet for large systems one wants a way to bring considerable manpower to bear, so that the product can make a timely appearance. How can these two needs be reconciled? Mills’s Proposal A proposal by Harlan Mills offers a fresh and creative solution. Mills proposes that each segment of a large job be tackled by a team, but that the team be organized like a surgical team rather than a hog-butchering team. That is, instead of each member cutting away on the problem, one does the cutting and the others give him every support that will enhance his effectiveness and productivity.
  14. Even though they have not taken centuries to build, most programming systems reflect conceptual disunity far worse than that of cathedrals. Usually this arises not from a serial succession of master designers, but from the separation of design into many tasks done by many men. I will contend that conceptual integrity is the most important consideration in system design. It is better to have a system omit certain anomalous features and improvements, but to reflect one set of design ideas, than to have one that contains many good but independent and uncoordinated ideas.
  15. The purpose of a programming system is to make a computer easy to use…Because ease of use is the purpose, this ratio of function to conceptual complexity is the ultimate test of system design. Neither function alone nor simplicity alone defines a good design. This point is widely misunderstood. As soon as ease of use is held up as the criterion, each of these is seen to be unbalanced, reaching for only half of the true goal. Ease of use, then, dictates unity of design, conceptual integrity. Conceptual integrity in turn dictates that the design must proceed from one mind, or from a very small number of agreeing resonant minds.
  16. Architecture must be carefully distinguished from implementation. As Blaauw has said, “Where architecture tells what happens, implementation tells how it is made to happen.”
  17. Not trivial, however, is the principle that such mini-decisions be made consistently throughout.
  18. In most computer projects there comes a day when it is discovered that the machine and the manual don’t agree. When the confrontation follows, the manual usually loses, for it can be changed far more quickly and cheaply than the machine.
  19. The project manager’s best friend is his daily adversary, the independent product-testing organization. This group checks machines and programs against specifications and serves as a devil’s advocate, pinpointing every conceivable defect and discrepancy. Every development organization needs such an independent technical auditing group to keep it honest.
  20. The second reason for the project workbook is control of the distribution of information. The problem is not to restrict information, but to ensure that relevant information gets to all the people who need it.
  21. The purpose of organization is to reduce the amount of communication and coordination necessary; hence organization is a radical attack on the communication problems treated above.
  22. The means by which communication is obviated are division of labor and specialization of function.
  23. On larger projects it is very rarely workable, for two reasons. First, the man with strong management talent and strong technical talent is rarely found. Thinkers are rare; doers are rarer; and thinker-doers are rarest.
  24. Practice is the best of all instructors. —PUBLILIUS
  25. Experience is a dear teacher, but fools will learn at no other. —POOR RICHARD’S ALMANAC
  26. The linear extrapolation of such sprint figures is meaningless. Extrapolation of times for the hundred-yard dash shows that a man can run a mile in under three minutes.
  27. Fostering a total-system, user-oriented attitude may well be the most important function of the programming manager.
  28. First, writing the decisions down is essential. Only when one writes do the gaps appear and the inconsistencies protrude. The act of writing turns out to require hundreds of mini-decisions, and it is the existence of these that distinguishes clear, exact policies from fuzzy ones.
  29. Chemical engineers learned long ago that a process that works in the laboratory cannot be implemented in a factory in only one step. An intermediate step called the pilot plant is necessary to give experience in scaling quantities up and in operating in nonprotective environments. For example, a laboratory process for desalting water will be tested in a pilot plant of 10,000 gallon/day capacity before being used for a 2,000,000 gallon/day community water system.
    1. Scaling Effects
  30. In most projects, the first system built is barely usable. It may be too slow, too big, awkward to use, or all three. There is no alternative but to start again, smarting but smarter, and build a redesigned version in which these problems are solved. The discard and redesign may be done in one lump, or it may be done piece-by-piece. But all large-system experience shows that it will be done. Where a new system concept or new technology is used, one has to build a system to throw away, for even the best planning is not so omniscient as to get it right the first time. The management question, therefore, is not whether to build a pilot system and throw it away. You will do that. The only question is whether to plan in advance to build a throwaway, or to promise to deliver the throwaway to customers. Seen this way, the answer is much clearer.
  31. Structuring an organization for change is much harder than designing a system for change.
  32. Things are always at their best in the beginning,” said Pascal. C. S. Lewis has stated it more perceptively: That is the key to history. Terrific energy is expended—civilizations are built up—excellent institutions devised; but each time something goes wrong. Some fatal flaw always brings the selfish and cruel people to the top, and then it all slides back into misery and ruin. In fact, the machine conks. It seems to start up all right and runs a few yards, and then it breaks down.
  33. A good workman is known by his tools. —PROVERB
  34. The most pernicious and subtle bugs are system bugs arising from mismatched assumptions made by the authors of various components.
  35. Many poor systems come from an attempt to salvage a bad basic design and patch it with all kinds of cosmetic relief. Top-down design reduces the temptation. I am persuaded that top-down design is the most important new programming formalization of the decade.
  36. Add one component at a time. This precept, too, is obvious, but optimism and laziness tempt us to violate
  37. Lehman and Belady offer evidence that quanta should be very large and widely spaced or else very small and frequent. The latter strategy is more subject to instability, according to their model. My experience confirms it: I would never risk that strategy in practice.
  38. How does one control a big project on a tight schedule? The first step is to have a schedule. Each of a list of events, called milestones, has a date. Picking the dates is an estimating problem, discussed already and crucially dependent on experience. For picking the milestones there is only one relevant rule. Milestones must be concrete, specific, measurable events, defined with knife-edge sharpness. It is more important that milestones be sharp-edged and unambiguous than that they be easily verifiable by the boss. Rarely will a man lie about milestone progress, if the milestone is so sharp that he can’t deceive himself. But if the milestone is fuzzy, the boss often understands a different report from that which the man gives.  Sharp milestones are in fact a service to the team, and one they can properly expect from a manager. The fuzzy milestone is the harder burden to live with. It is in fact a millstone that grinds down morale, for it deceives one about lost time until it is irremediable. And chronic schedule slippage is a morale-killer.
  39. The preparation of a PERT chart is the most valuable part of its use. Laying out the network, identifying the dependencies, and estimating the legs all force a great deal of very specific planning very early in a project. The first chart is always terrible, and one invents and invents in making the second one.
  40. Most of the big last gains in software productivity have come from removing artificial barriers that have made the accidental tasks inordinately hard, such as severe hardware constraints, awkward programming languages, lack of machine time. How much of what software engineers now do is still devoted to the accidental, as opposed to the essential? Unless it is more than 9/10 of all effort, shrinking all the accidental activities to zero time will not give an order of magnitude improvement. Therefore it appears that the time has come to address the essential parts of the software task, those concerned with fashioning abstract conceptual structures of great complexity. I suggest:
    1. Exploiting the mass market to avoid constructing what can be bought.
    2. Using rapid prototyping as part of a planned iteration in establishing software requirements.
    3. Growing software organically, adding more and more function to systems as they are run, used, and tested.
    4. Identifying and developing the great conceptual designers of the rising generation.
  41. The gap between the best software engineering practice and the average practice is very wide—perhaps wider than in any other engineering discipline. A tool that disseminates good practice would be important.
  42. The development of the mass market is, I believe, the most profound long-run trend in software engineering. The cost of software has always been development cost, not replication cost. Sharing that cost among even a few users radically cuts the per-user cost. Another way of looking at it is that the use of n copies of a software system effectively multiplies the productivity of its developers by n. That is an enhancement of the productivity of the discipline and of the nation.
  43. No other part of the conceptual work is so difficult as establishing the detailed technical requirements, including all the interfaces to people, to machines, and to other software systems. No other part of the work so cripples the resulting system if done wrong. No other part is more difficult to rectify later. Therefore the most important function that software builders do for their clients is the iterative extraction and refinement of the product requirements. For the truth is, the clients do not know what they want. They usually do not know what questions must be answered, and they almost never have thought of the problem in the detail that must be specified.
  44. I would go a step further and assert that it is really impossible for clients, even those working with software engineers, to specify completely, precisely, and correctly the exact requirements of a modern software product before having built and tried some versions of the product they are specifying. Therefore one of the most promising of the current technological efforts, and one which attacks the essence, not the accidents, of the software problem, is the development of approaches and tools for rapid prototyping of systems as part of the iterative specification of requirements. A prototype software system is one that simulates the important interfaces and performs the main functions of the intended system, while not being necessarily bound by the same hardware speed, size, or cost constraints. Prototypes typically perform the mainline tasks of the application, but make no attempt to handle the exceptions, respond correctly to invalid inputs, abort cleanly, etc. The purpose of the prototype is to make real the conceptual structure specified, so that the client can test it for consistency and usability.
  45. Incremental development—grow, not build, software. I still remember the jolt I felt in 1958 when I first heard a friend talk about building a program, as opposed to writing one. In a flash he broadened my whole view of the software process. The metaphor shift was powerful, and accurate. Today we understand how like other building processes the construction of software is, and we freely use other elements of the metaphor, such as specifications, assembly of components, and scaffolding. The building metaphor has outlived its usefulness. It is time to change again. If, as I believe, the conceptual structures we construct today are too complicated to be accurately specified in advance, and too complex to be built faultlessly, then we must take a radically different approach. Let us turn to nature and study complexity in living things, instead of just the dead works of man. Here we find constructs whose complexities thrill us with awe. The brain alone is intricate beyond mapping, powerful beyond imitation, rich in diversity, self-protecting, and self-renewing. The secret is that it is grown, not built. So it must be with our software systems. Some years ago Harlan Mills proposed that any software system should be grown by incremental development. That is, the system should first be made to run, even though it does nothing useful except call the proper set of dummy subprograms. Then, bit by bit it is fleshed out, with the subprograms in turn being developed into actions or calls to empty stubs in the level below. I have seen the most dramatic results since I began urging this technique on the project builders in my software engineering laboratory class. Nothing in the past decade has so radically changed my own practice, or its effectiveness. The approach necessitates top-down design, for it is a top-down growing of the software. It allows easy backtracking. It lends itself to early prototypes. Each added function and new provision for more complex data or circumstances grows organically out of what is already there. The morale effects are startling. Enthusiasm jumps when there is a running system, even a simple one. Efforts redouble when the first picture from a new graphics software system appears on the screen, even if it is only a rectangle. One always has, at every stage in the process, a working system. I find that teams can grow much more complex entities in four months than they can build. The same benefits can be realized on large projects as on my small ones. 
  46. The differences are not minor—it is rather like Salieri and Mozart. Study after study shows that the very best designers produce structures that are faster, smaller, simpler, cleaner, and produced with less effort. The differences between the great and the average approach an order of magnitude.
  47. My first proposal is that each software organization must determine and proclaim that great designers are as important to its success as great managers are, and that they can be expected to be similarly nurtured and rewarded. Not only salary, but the perquisites of recognition—office size, furnishings, personal technical equipment, travel funds, staff support—must be fully equivalent. How to grow great designers? Space does not permit a lengthy discussion, but some steps are obvious:
    1. Systematically identify top designers as early as possible. The best are often not the most experienced.
    2. Assign a career mentor to be responsible for the development of the prospect, and keep a careful career file.
    3. Devise and maintain a career development plan for each prospect, including carefully selected apprenticeships with top designers, episodes of advanced formal education, and short courses, all interspersed with solo design and technical leadership assignments.
    4. Provide opportunities for growing designers to interact with and stimulate each other.
  48. Turski and I both insist that pipe-dreaming inhibits forward progress and wastes effort.
  49. Capers Jones, writing first in a series of memoranda and later in a book, offers a penetrating insight, which has been stated by several of my correspondents. “NSB,” like most writings at the time, was focused on productivity, the software output per unit of input. Jones says, “No. Focus on quality, and productivity will follow.” He argues that costly and late projects invest most of the extra work and time in finding and repairing errors in specification, in design, in implementation. He offers data that show a strong correlation between lack of systematic quality controls and schedule disasters. I believe it.
  50. Representation is the essence of programming.
  51. Fixing a defect has a substantial (20 to 50 percent) chance of introducing another.
  52. Vyssotsky: “I have found it handy to carry both ‘scheduled’ (boss’s dates) and ‘estimated’ (lowest-level manager’s dates) dates in the milestone report. The project manager has to keep his fingers off the estimated dates.”
  53. The subsystem boundaries must be at those places where interfaces between the subsystems are minimal and easiest to define rigorously.
  54. Featuritis. The besetting temptation for the architect of a general purpose tool such as a spreadsheet or a word processor is to overload the product with features of marginal utility, at the expense of performance and even of ease of use. The appeal of proposed features is evident at the outset; the performance penalty is evident only as system testing proceeds. The loss of ease of use sneaks up insidiously, as features are added in little increments, and the manuals wax fatter and fatter.
  55. If one believes, as I have argued at many places in this book, that creativity comes from individuals and not from structures or processes, then a central question facing the software manager is how to design structure and process so as to enhance, rather than inhibit, creativity and initiative. Fortunately, this problem is not peculiar to software organizations, and great thinkers have worked on it. E. F. Schumacher, in his classic, Small is Beautiful: Economics as if People Mattered, proposes a theory of organizing enterprises to maximize the creativity and joy of the workers. For his first principle he chooses the “Principle of Subsidiary Function” from the Encyclical Quadragesimo Anno of Pope Pius XI: It is an injustice and at the same time a grave evil and disturbance of right order to assign to a greater and higher association what lesser and subordinate organizations can do. For every social activity ought of its very nature to furnish help to the members of the body social and never destroy and absorb them. . . . Those in command should be sure that the more perfectly a graduated order is preserved among the various associations, in observing the principle of subsidiary function, the stronger will be the social authority and effectiveness and the happier and more prosperous the condition of the State.

What I got out of it

  1. The importance of thinking in parallel vs. series, adding margins of safety (things always go wrong, so do you bake that into your assumptions or do you pay for it dearly at a later point?), adding more software developers generally makes projects even later, as few minds as possible to make the system easy to use (top-down design one of the most important aspects to consider), the importance of testing and iteration at every step along the process (grow, don’t build software), sharp rather than fuzzy milestones

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