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Now that we saw what the main values and practices

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of XP are, I want to go back for a minute

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to discussion of requirements engineering in XP. In XP, user

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requirements are expressed as scenarios or user stories, as we already

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discussed. These are written by customers on cards, and what

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the development team does is to take these cards, take these

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users stories and break them down into implementation tasks. And

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those implementation tasks are then used as a basis for scheduling

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cost estimates. So given these estimates, and based on their priorities,

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the customer will choose the stories that will be included in

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the next release, in the next iteration. And at this point,

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the corresponding cards will be taken by the developers and the,

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the task will be performed, and the relative, and the corresponding

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card will be developed. And just to give an idea of

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the order of magnitude, if you consider a few months project,

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there might be 50 to 100 user stories for a project of

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that duration. So, now let me give you an example of what

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the story card might look like, and I'm going to do it using

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a story card for document downloading and you can really do all of

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this, basically as seeing what the

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scenario is, downloading and printing an article.

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And it describes basically what happens when you do that, what is

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the scenario. First, you select the article that you want from a displayed

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list. You then have to tell the system how you will pay for

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it. This can either be through a subscription, through a company account or

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by credit card, and so on. So what developers do, they take

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this story card, and they break it down in to development tasks.

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So, here I'm showing you some examples of task cards for the

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user story that we just saw. In particular I'm showing three task cards

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and if we look at the third one, there is a name

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for the task, which is implement payment collection. So this is the development

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task that we have the perform and here, there's a description of

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what that developed code should do. And notice that, you know, the task

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card can even be more. explicit than this, more

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specific than this, and talk about actual development tasks.