hát triển sản phẩm đúng theo yêu cầu của khách hàng: thực tế cho thấy rằng nhiều sản phẩm phần mềm tuy được phát triển một cách công phu nhưng lại không đáp ứng được nhu cầu của người sử dụng. Phương pháp XP đã đưa ra các cơ chế cho phép sản phẩm phát triển luôn phù hợp với yêu cầu của người sử dụng. | 66 Chapter 12 Four Variables We use four variables to help us think about how to control a project cost quality time and scope. They are inter-related and effect each other in ways that are not obvious. We ve all heard statements like cost time quality pick any two . Plenty of people have ways in which they talk about how there are these variables involved in getting something done and that you can t control them all at once. Well we choose a set of variables too and we find them a good set to work with. They are Cost Quality Time Scope We like to think of them as four levers on some big victorian steam machine. The four levers control the machine which is our project of course . If you move any lever the others move. You can lock any lever you like but if you lock three levers you cannot move the fourth. The catch however is that the effect of moving a lever is both delayed and non-linear. You can t just double the cost hold everything else the same and halve the time. So each lever gets it s own little instruction manual. The good news is that the manual wasn t written by a second rate victorian novelist. Cost If you look at the cost lever carefully you quickly see that it s actually several mostly independent levers. Moving any of them can increase or reduce your costs but each lever has a different effect on the three other primary levers. The most powerful lever is that of people. You increase this lever by putting more people on the project. This lever however suffers from having both a non-linear effect and a long delay. The non-linearity comes from the communication overhead of having more people. Doubling your team doesn t make you go twice as fast because it increases the amount of communication that needs to go on. There isn t really much guidance we can give you on this partly because there isn t the data and partly because so many other factors have an effect. All you can do is add some people and see the effect. The trouble is you ll have to wait to .
