That is one of the odd things about this term. Originally, it was indeed about three constraints: Cost, Time, and Scope – project and programme managers have to deliver the goods (Scope), within the budget (Cost), within the schedule (Time).
Later, thinkers thought to add 'Quality' because well, sure, you can deliver the goods, but are the goods you deliver up-to-spec?. So the Triple Constraints began to consist of four items. There are some heretics (or perhaps they are prophets of wisdom) who insist that Quality is just part of Scope, and ought not be considered separately from the other. The argument is, if you delivered 600 green umbrellas rather than the required 600 red umbrellas, it is not a 'Quality' problem but a Scope problem – you simply didn't deliver what was asked for.
Project managers with more than a week experience, began to realise that despite their personal charm, the world doesn't revolve around them or their naive assumptions.
Things go wrong.
"We need to add another constraint - Risk", came the chorus. And so the Triple Constraints became five (and have to be redrawn as a pentagon – we are not geometrical ignorami, after all)
Courageous project and programme managers soon began to point out their specialty: delivering goods within the constraints of time, cost, scope, quality, and risks. There is no need to be mean-spirited by reminding them that postmen and newspaper delivery kids and mothers and housewives, have been doing the same since the invention of anything.
But no matter how well we think we manage the five constraints, projects still tend to head south, and so we need another constraint to lay the blame on.
Resources.
"Surely Resources are a constraint!" came the battle-cry of the project and programme manager. "Even if I have a budget of $50 million if I cannot find Fortran programmers, my Fortran project cannot succeed." And of course, when they burn up my $50 million without delivering anything, they can point to the incompetent resources they had to work with.
But let's take a step back (after first checking we are not on the edge of a cliff; risk management in action).
We can take the view that the constraints are about what a project manager has to deliver against. Or we can take the view that the constraints are 'natural' limitations that enterprises have to balance when deciding which projects to undertake.
We want to finish the project as soon as possible so we can reap the benefits and start other projects as well. But to finish faster means applying more resources. This is a natural constraint in our world.
We want to do it right the first time, but it will mean more time and more cost, so we may need to cut a little bit of scope here and there. Another natural constraint.
We want to deliver the project as cheaply as possible, but if we put a limited number of resources will take the project that much longer to finish. Another natural constraint.
I think the Triple Constraints are about having the enterprise (not the project manager) balance between the 3 (not six) constraints. The project manager, for his part have to deliver the Triple Constraints while working through the constraints of Risks and Resources. (Quality is part of Scope).
