The Triple Constraint

Here's a bit of elementary project management: the triple constraint. This was explained to me by an experienced project manager colleague some time ago, who drew a diagram similar to the one below on my whiteboard.

The "Q" in the middle stands for quality. The point is that the variables are interrelated: changing one affects both of the others. If you don't account for the changes to the others, you "break the triangle" and it's quality that suffers.

Now I don't really have any formal project management experience, but I've been on a lot of projects. I've been on projects where the end date and the size of the team ("cost") were fixed, but scope was increased. And what do you know, when we got to the end date, there was a huge pile of bugs. When my colleague presented this model to me it seemed blindingly obvious. Yet, it seems that many people just don't get it.

(Some literature considers "triple constraint" to be an obsolete term, preferring instead to consider cost, quality, scope, and time as a set of four variables. This goes against the Received Wisdom of Quality, which states that Quality shall not be a variable. But I digress.)

Upon further reflection, it's not that people don't "get" the triple constraint. The real problem is that people are unwilling to confront the results of thinking through the model.

Now, like any model, there are some qualifications. I'm thinking of two in particular: "in the long run" and "all other things being equal." I'll cover these in separate blog posts.

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