Excerto do livro eXtreme Programming Explained, do Kent Beck, sobre as quatro principais variáveis de controlo num projecto:

Cost — More money can grease the skids a little, but too much money too soon creates more problems than it solves. On the other hand, give a project too little money and it won’t be able to solve the customer’s business problem.

Time — More time to deliver can improve quality and increase scope. Since feedback from systems in production is vastly higher quality than any other kind of feedback, giving a project too much time will hurt it. Give a project too little time and quality suffers, with scope, time, and cost not far behind.

Quality — Quality is terrible as a control variable. You can make very short-term gains (days or weeks) by deliberately sacrificing quality, but the cost—human, business, and technical—is enormous.

Scope — Less scope makes it possible to deliver better quality (as long as the customer’s business problem is still solved). It also lets you deliver sooner or cheaper.

There is not a simple relationship between the four variables. For example, you can’t just get software faster by spending more money. As the saying goes, “Nine women cannot make a baby in one month.” (And contrary to what I’ve heard from some managers, eighteen women still can’t make a baby in one month.)

Eu diria que é nada mais que senso comum, mas quando dito de uma forma tão simples e clara aproxima-se de genial!