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+So, let's start with requirements engineering, which is the
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+field within software engineering that deals with establishing the
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+needs of stakeholders that are to be solved by
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+the software. So why is this phase so important?
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+In general, the cost of correcting an error depends
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+on the number of subsequent decisions that are based
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+on it. Therefore, errors made in understanding requirements have
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+the potential for greatest cost because many other design decisions
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+depend on them and many other follow up decisions depend on them.
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+In fact, if we look at this diagram, which is again a
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+qualitative diagram, where we have the cost of error correction over the
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+phase in which the error is discovered. We can see that if we
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+discover an error in requirements it's going to cost us one. If
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+we find it in in design it's going to cost us five and
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+so on and so forth. And the cost grows dramatically as we
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+go from the requirements phase to the maintenance phase. Why? Because of course
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+if we discover a problem here we're left to undo a
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+lot of the decision that we had made before to correct the
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+error. Whereas if we find an error here we can correct it
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+right away and we don't affect the subsequent phases. So how can
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+we collect the right requirements. Traditional
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+requirements in engineering does so through
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+a set of steps. The first step is elicitation which is the
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+collection of requirements from stake holders and other sources and can be
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+done in a variety of ways, we will discuss some of them.
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+The second is requirement analysis which involved the study and
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+deeper understanding of the collective requirements. The third step is this
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+specification of requirements, in which the collective requirements are suitably
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+represented, organized and save so that they can be shared. Also
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+in his case, there are many ways to do this,
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+and we will see some of this ways when we talk
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+about the requirements engineering in the dedicated lesson. Once the
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+requirements have been specified, they can be validated to make sure
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+that they're complete, consistent, no redundant and so on. So
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+that they've satisfied a set of importance properties, for requirements.
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+Finally, the fifth step is requirements management which accounts for
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+changes to requirements during the lifetime of the project. And here
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+I talked about steps, kind of giving the impression that
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+we're just going from the first step to the fifth one
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+and that this is sort of a linear process. In
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+reality, as we will see, this is more of an iterative
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+process in which will go and cover the different phases in an
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+iterative fashion. We will discuss extensively
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+requirements engineering in our second mini-course.