At its heart, successful project management requires softer skills such as communication and persuasion. This why the few more mathematical elements that do exist within project management receive such attention, from estimation techniques to Gantt charting and the triple constraint. However, unfortunately the triple constraint is more of an optical illusion than an effective representation of the project management trade-offs.
Let’s start with cost. The triple constraint is inconsistent with Frederick Brooks’ insights. If you add money to a project, you are more likely than not going to have to add people. Those people will need training and expand the communication nexus across the project team exponentially. This additional training and communication will slow things down, at least in the short term.This is the opposite of what the triangle supposes. So you cannot seamlessly flex the budget to ensure you hit your project goals.
Next scope, the problem here is representing scope as a continuum, where you can chip away or incrementally add work items to the project to flex schedule and cost to the right values. This approach works until it doesn’t. Project success is often binary rather than a continuum. The iPhone 4’s reception problems or the Deepwater Horizon rig show that only up to a point can scope scale up and down before the project hits major risks. In fact, the baggage system at Denver Airport shows that the scope of some projects are infeasible regardless of cost or time.
That leaves schedule as the one corner of the project triangle that can legitimately flex, perhaps this accounts for why most projects finish late. The other corners of the triangle aren’t as malleable as they seem.