Ihad an interesting question about plan reliability. "Why does reliability (PPC) matter?" My first thought was, "Where do I start? Of course it matters!" Ok. Breathe in; breathe out. I know what to do.
A good planning system will enable project team members to fulfill their promises just as they make them
Let's start with PPC. We recommend measuring planning reliability using the measurement percent of promises completed. Our thinking is if people can do what they promise to do, then the planning is good. It doesn't mean that the future should be just as we planned it to be. Life's not like that. But, if we're doing a really good job with our planning, then most of the promises we make for completing work will be kept. PPC is a measure of reliability.
Reliability is an indirect measurement of the quality of the planning system. One of our goals of planning is to accomplish what we set out to do. Another way of saying this is, "I promise to get "x" done by time "y"." A good planning system will enable project team members to fulfill their promises just as they make them. Is this important? (I'm thinking to myself, "Hell yeah!") Reliable completion of our promises releases work for other performers. By keeping my promise, the next person in line is able to keep their promise. And so on, and so on. That is the nature of projects.
When task completion is unreliable, then the completion of the project is indeterminable
Eli Goldratt tells an instructive story in The Goal on the nature of the compounding of dependence with variation. It's a little game (using match sticks) that shows that when one action depends on a prior action that variance in the performance of those actions leads to an unpredictable completion of the chain of actions (covariance for the Six Sigma folk). In other words, when task completion is unreliable, then the completion of the project (chain of tasks) is indeterminable. That's why reliability matters. Being reliable on a task-by-task basis is the only way we can get our projects done on time.