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+The next model that we will discuss is the spiral
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+model, which was first described by Barry Boehm, which is
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+the professor that we interviewed at the beginning of this
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+lesson. In his paper from 1986 that was entitled A Spiral
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+Model of Software Development and Enhancement. And one of the
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+main characteristics of that paper is that it was describing the
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+spiral model using a diagram, which is the one that
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+I'm showing you here, and this diagram has become very very
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+popular, and you probably saw it either in this
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+form or one of the many variations of the
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+diagram. So I'm not going to discuss all of
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+the details of the spiral model, but I just want to
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+give you an idea of its main characteristics. The
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+spiral model is an incremental risk-oriented lifecycle model that has
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+four main phases listed here: determine objectives, identify and
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+resolve risks, development and tests, and plan the next iteration.
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+A software project will go through these four phases in
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+an iterative way. In the first phase, the requirements will
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+be gathered. In the second phase, the risks and the
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+alternate solutions will be identified, and a prototype will be produced.
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+Software and tests for the software are produced in the
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+development and test phase, which is the third step of the
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+process. Finally, in the fourth phase, the output of the
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+project, so far, is evaluated, and the next iteration is planned.
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+So basically, what the spiral process prescribes is a
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+way of developing software by going through these phases in
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+an iterative way, in which we learn more and more
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+of the software, we identify more and more, and account
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+for, more and more risks and we go more
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+and more towards our final solution, our final release. There
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+are several advantages of using a spiral model. The first
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+one is that the extensive risk analysis does reduce the
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+chances of the project to fail. So there is a
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+risk reduction advantage. The second advantage is that functionality can be
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+added at a later phase because of the iterative nature of
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+the process. And finally, software is produced early in the software
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+lifecycle. So, at any iteration, we have something to show
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+for our development. We don't wait until the end before producing
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+something. And then of course there's also the advantage that we
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+can get early feedback from the customer about what we produced.
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+The main disadvantages on the other hand of
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+the spiral model, are that the risk analysis requires
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+a highly specific expertise. And unfortunately, the whole success
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+of the process is highly dependent on risk analysis.
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+So risk analysis has to be done right.
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+And finally the spiral model is way more complex
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+than other models, like for example, the water fall
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+model. And therefore it can be costly to implement.