The consideration of the value, or worth, of a project objective always takes into account three "dimensions":

Performance - What is the deliverable?
Timeliness - When will the project be complete?
Cost - What is the total cost, including any hidden costs, of the project?
(All cost ultimately reduces to effort.)

The Performance dimension to a project objective describes the deliverable - the target and tolerances (i.e. 'quality') of what will be produced or changed.

Now, ends imply means. To have a desired result (e.g. piano skills) requires that one do certain things. (Practice!) Given a particular project strategy, "performance" or "deliverables" determines what actions (tasks) must be accomplished in order to complete the project.

Any single task that is performed correctly takes "as long as it takes", and "costs what it costs". Therefore, many people assume that since the performance dimension implies tasks, it also implies a project timeframe and a project cost.

By this line of thinking, there is only one independent dimension to a project objective; the other dimensions cannot freely vary. Many project management texts correctly disagree with this assessment.

If you consider the fact that projects are composed of multiple concurrent tasks; many authors observe that, if deliverables are held constant, many tradeoffs between project completion times and costs are possible.

One tradeoff is extend the deadline. Extending the timeline creates opportunities for efficiency. The astute manager seizes these opportunities and reduces costs. The alternative is to push for an early completion by paying expediting fees, overtime, etc..

Unfortunately, over time this has been oversimplified to the slogan, "You can have two of the three". Meaning that Performance, Time & Cost form an unyielding "iron triangle" with only two independent dimensions. (Or, "degrees of freedom"). Additional deliverables always must be paid for by a later delivery time or by an increased cost.

This "iron triangle" view of the three dimensions to project objectives is only true in the simplest of projects, and is most often used as an excuse for poor project management.

There are at least three reasons not to subscribe to the "iron triangle" view of performance-time-cost.

First, consider that it is always possible to spend more time and more money on any project without obtaining the slightest improvement in deliverables!

This thought should at least prepare you to consider the notion that deliverables may often be achieved with less time and money

This achievement requires active management! It is a fact of life that the project environment will change. The fundamental tactical skill is the ability to detect significant change and to appropriately redeploy resources.

Estimating handbooks and "comparables" estimates will inevitably 'crank' a bit of cushion into the costs, as they are based on averages. It is the standard deviation in these statistics that active management will exploit. 

Good project management requires that the essential management tools be used. Task plans, Resource loading analysis, good purchasing and accounts payable skills, critical path calculations, and project tracking skills all facilitate the best use of time and money to achieve a deliverable.

If the project manager is not maintaining the kind of continuous focus necessary to obtain optimum results, then anyone who does will certainly achieve a more desirable blend of performance-time-cost.

Second, the "equilibrium", or 'maximum payback' balance point of Performance-Time-Cost is extremely sensitive to project strategy.

Imagine a project where the first  sub-objective is: "Bring up an "empty" web site, ready for content."

I could: a) Buy hardware ($2000), b) connect to the internet ($90/month), c) load software (8 hours @ $50/hour).

Or, for less than $400 per year, and 30 minutes on-line - I could bring up the empty web site on a commercial service - who will even include perks like: backup services, maintenance, redundant connections to the internet and to power for high reliability, and physical security services.

Of course, project strategy is not entirely in the "say so" of the project manager. Responsibility for Strategic decision-making flows from the customer. (Capital S Strategic) Most often, in complex technical projects the project architect will referee these issues.

Still, project strategy is at least partially within the realm of the project manager's authority. Because of this, the "iron triangle" concept applies only in the unlikely context where every tactical advantage has been seized, and there are no strategic decisions left on the table, and when all the possibilities of a third factor have been exhausted!

The Third Factor. There is another dimension the the equation. The basic formula -performance-time-cost is the essence of the value function. When we appraise a (net) value, we ask: 1) What are we going to get? 2) When will we get it? 3) What will it cost?

There is a fourth dimension and it is Risk.

It used to be said that "no one was ever fired by buying IBM". The company used to be the undisputed giant in automation. Customers often paid a heavy premium for products and services from what consensus said was a 'low risk' vendor.

Maverick managers bought 'riskier' products from smaller companies that lacked the IBM brand strength. When they succeeded, their projects beat the 'iron triangle' of conventional wisdom.

Now, do not rush straightaway to the bargain shop just yet. In large numbers risk is real. Unmanaged, the incorporation of products and vendors of lesser and unknown reputation ('riskier'') will in one case raise costs while costs are lowered in another.

This secret to getting more in less time for less money lies in your ability to manage risk.

The two fundamental issues here are:

1) Where in the project does not use the so-called 'high risk' vendor?

2) When a 'high-risk' vendor or product is considered, can we exactly identify the nature of the risk and mitigate it?

The stubborn defenders of the Iron Triangle philosophy are often risk-averse project managers who 'always buy IBM', but every project of any size has a 'risk' dimension.

The essence of risk is uncertainty. One cannot arrange a project so that one need not ask. 'Am I certain? How do I know?" The successful project manager will recognize and embrace the management of risk, and break the Iron Triangle.