The four variables of software development projects in eXtreme Programming. The four variables of software development controlling by XP seeks to minimize risk which are:
Cost is probably the most constrain. You cannot spent your way to schedule or quality and you have very limited control over cost as a developer. Also Cost is where Brook’s law gets invoked.
Usually, Time is unfortunately imposed on you form the outside and is your delivery schedule. For example, most consumer products will have early fall in order to hit the holiday buying season or a delivery date in late summer.
The only way to fix your problem is to lessen quality or drop features, if you are late; non-of which is pretty. Features are what the product actually does. It is what developers should always focus on. You as a developer it is also the one have the most control-over. From customer’s perspective it is the most important of the variables.
Quality is the severity and number of defects you are willing to deliver with. In delivery schedules you can make short term gains by sacrificing quality, but the cost is enormous. Your credibility is pretty well shot. It will take more time to fix the next delivery.
Controlling scope permit you to give customers and managers control over cost, time and quality. XP recognizes that developers need to control as many of the variables as possible, to minimize risk. Especially, they need to control the project scope. “Learning to drive is metaphor XP uses. The car in the right way learning to drive is not pointing. Constantly, It is pointing the car, making the constant minor corrections and paying attention necessary to keep the car on road. The only constant is change in programming. You can keep cost of change manageable, if you pay attention and scope with change as it occurs.