an article on “scope,” “time,” and “cost” and project management tradeoffs by John Jan Popovic

Project Management Triangle
Project Management Triangle
- TIME
- BUDGET
- SCOPE: all feature set with desired quality
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Chose ONE of three TO BE SACRIFICED!

The "Scope Triangle" or the "Quality Triangle" shows the trade-offs inherent in any project. At least ONE of three often MUST BE SACRIFICED!
Like any human undertaking, projects need to be performed and delivered under various constraints. Usually these constraints have been listed as “scope,” “time,” and “cost”. Each side represents a constraint. One side of the triangle cannot be changed without affecting the others. A further refinement of the constraints separates product “quality” or “performance” from scope, and turns quality into a fourth constraint.
More functions (scope) one application has the QUALITY is higher!
Usually more time is used to write and refactor the code the QUALITY is higher!
Usually more money is invested the code the QUALITY is higher!
The triangle illustrates the relationship between three primary forces in a project. Time is the available time to deliver the project, cost represents the amount of money or resources available and quality represents the fit-to-purpose that the project must achieve to be a success.
Usually one of these factors is fixed and the other two will vary in proportion. For example time is often fixed and the quality of the end product will depend on the cost or resources available. Similarly if you are working to a fixed level of quality then the cost of the project will largely be dependent upon the time available, if you have longer you can do it with fewer people.
A project manager is the person accountable for accomplishing the stated project objectives. Project manager can have the responsibility of the planning, execution, and closing of any project.
Key project management responsibilities include creating clear and attainable project objectives, development plan road map, building the project requirements, and managing the triple constraint for projects, which is cost, time, and scope.
A project manager is often a client representative and has to determine and implement the exact needs of the client, based on knowledge of the firm they are representing. The ability to adapt to the various internal procedures of the contracting party, and to form close links with the nominated representatives, is essential in ensuring that the key issues of cost, time, quality and above all, client satisfaction, can be realized.
The time constraint refers to the amount of time available to complete a project. The cost constraint refers to the budgeted amount available for the project. The scope constraint refers to what must be done to produce the project’s end result. These three constraints are often competing constraints: increased scope typically means increased time and increased cost, a tight time constraint could mean increased costs and reduced scope, and a tight budget could mean increased time and reduced scope.
The clever reader will be wondering what happens when two of the points are fixed. This is when it really gets interesting. Normally this occurs when costs are fixed and there is a definite deadline for delivery, an all too familiar set of circumstances. Then, if the scope starts to creep you are left with only one choice - cut functionality. This more common than you might think, in fact its more common than not!
Cutting functionality may seem a drastic measure, but an experienced project manager will happily whittle away functionality as if they were peeling a potato. As long as the core requirements remain, everything will be fine. Additional functionality can always go into "the next release," but if you don't deliver the core functionality, there won't be a next release.
A really experienced project manager might even include in his project with a little superfluous functionality that could be sacrificed when the crunch comes, and it will come sooner or later.
A phenomenon known as "scope creep" can be linked to the triangle too. Scope creep is the almost unstoppable tendency a project has to accumulate new functionality. Some scope creep is inevitable since, early on, your project will be poorly defined and will need to evolve. A large amount of scope creep however can be disastrous.

Often in software development you can NOT convert money to time by hiring more programmers; often there isn't the time for that (recruitment and understanding of the requirements takes time), which is why cost and time are separately listed.
Sometimes you might consider varying scope, leaving only some essential features. You can say then that the fixed features are top priority, and the additional features are of secondary importance.
Eg1 Time and errors in a software development
E.g. writing the software with "strict" and "warnings" constraints: code standard is fixed (you must respect all coding quality constraints); on the other hand -- time is fixed (no money, you have to do production code as soon as possible - quick and dirty code); the tradeoff is between writing the software fast or doing it carefully.

Eg2 Quality first. Sydney opera house; writing Fawlty Towers. Things you've done often have fairly well known standard time and costs; outstandingly creative things have unknown time and costs. The opera house ran over budget by about 10 times in both time and money, but the quality left the city with a landmark that is recognised round the world from a glimpse.

Eg3 John Cleese, famous English actor, writer and film producer once said it took them about 3 times longer to write the Fawlty Towers scripts than the standard BBC allowance (in time and money); but these are the shows that are still celebrated decades later, while others done on time and to budget were never re-run. These are cases of putting quality first.

Eg4 The Boeing Dreamliner programme, announced in 2003, was supposed to cost $6 billion and see the plane take to the air in 2008. The final bill was $32 billion and the 787 Dreamliner arrived three years late. The result of a combination of various technical failures and supply-chain chaotic mess. With engineers, designers and other resources diverted into saving the Dreamliner production plans, the rest of its production were delayed.

When a project (e.g. a road bridge) involves safety, it often means safety-quality has to be put first (or the project cancelled without completion).