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Extreme Programming Explained by Kent Beck

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Chapter 4. Four Variables

We will control four variables in our projects—cost, time, quality, and scope. Of these, scope provides us the most valuable form of control.

Here is a model of software development from the perspective of a system of control variables. In this model, there are four variables in software development:

  • Cost

  • Time

  • Quality

  • Scope

The way the software development game is played in this model is that external forces (customers, managers) get to pick the values of any three of the variables. The development team gets to pick the resultant value of the fourth variable.

Some managers and customers believe they can pick the value of all four variables. "You are going to get all these requirements done by the first of next month with exactly ...

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