The concept of an Iron Triangle has been around awhile. Cost, Schedule, Quality - in the iron triangle are related. This of course has been the brunt of the anti-PMBOK movement for awhile.
In the current defense and space domain and context, there are three variables that replace the Iron Triangle with a triangle that is connected:
- Cost - what is the budget allocated for the work to be performed?
- Schedule - what is the time allocated for this work. The Period of Performance for the Work Packages, Rolling Wave they make up and the Planning Packages for the remaining work?
- Technical Performance Measures - what is the target technical performance for the work product produced by the allocated cost (BCWS) applied to the Work Packages over the planned period of performance?
These three variables are immutable. If you change one the other two may or may not change. This depends on the coupling between them. They are not Iron.
The current PMBOK dropped this notion, but failed to replace it with an understanding of the coupling between Cost, Schedule, Technical Performance.
When you're thinking of the performance of a project, all three variables must be considered. This is the current assessment process for government programs. This is the core approach to Earned Value Management,
- Measure the physical percent complete against the planned cost to produce the Earned Value
- Assess the compliance of the delivered product
Since the units of measure of EV are Dollars, time and technical performance targets must be evaluated through some form of calculation. The time aspects is the Schedule Performance Index. But the TPM must be predefined before the work starts and assessed at the conclusion of the period of performance for the Work Package.
It's this predefined value that is commonly missing in projects. "What does done look like," needs to be defined BEFORE the work starts, so you can recognize it - or its absence - when you reach an assessment point.
