Four Variables: Cost, Time, Quality, Scope

4 variables: Cost, Time, Quality, Scope. Cost has diminishing returns. Delaying Time causes coordination problems. Reducing Quality backfires quickly. Controlling Scope is your best option.

There is a concept in project management called the “iron triangle”.

The Iron Triangle describes the need to trade-off against 3 constraints: Cost, Time, Scope. You can only control 2 out of 3.

However, you may notice that project managers / customers / clients will still try to control all 3 constraints by sacrificing an implicit 4th constraint: Quality.

So instead, there is a model from Extreme Programming called the Four Variables: Cost, Time, Quality, Scope.

Cost. Manipulating cost to accelerate delivery has diminishing returns. There are some quick wins here with faster machines and better equipment. After that, cost ends up being about adding people which generally does not help in the short term (see Mythical Man Month). There are some tactics, like pairing and mobbing, that help with absorbing people more effectively, but even they are not perfect.

Time. Delaying schedule to accommodate scope generally causes broader coordination problems. If there are any dependencies on a delivery, delays tend to cascade throughout a program or organisation, that is, I’m late and you depend on me, so you’re late and they depend on you so they’re late, etc. This is assuming that schedule delay is even available as an option and not essentially fixed due to external constraints like legislation, timing of holidays, etc.

Quality. Sacrificing quality to accelerate delivery tends to backfire much more quickly than most people expect. “Quality” here does not refer to the external qualities of features, polish, scalability, etc. but rather the internal quality of the product that affects our ability to continue to modify it. In fact, maintaining and improving internal quality is more likely to increase the likelihood of delivery of Cost, Time, Scope, than it is to decrease it (aka “Quality is free”).

Scope. Scope is the most valuable and predictable variable to control. Most deliveries specify much more in terms of features and external quality than are actually needed to deliver the needed outcomes. It is easier to coordinate knowing that someone will deliver at least a minimal version by a particular date, with the possibility of more, than to not know if anything will be delivered.