Exploration Through Example

Example-driven development, Agile testing, context-driven testing, Agile programming, Ruby, and other things of interest to Brian Marick
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Tue, 09 Nov 2004

Adding a basement to the house

Agile methods people claim changed requirements late in a project are not a disaster. Skeptics claim that's impossible, that it's like finishing the first story of a house and then deciding you want a basement.

That's a misguided analogy. The reason putting in a basement after the walls are up is hard is because almost no one does it. If it was done to every house during construction, you may be sure that homebuilders would have learned to do it as cheaply as is physically possible.

Agile projects don't think ahead: in iteration N, they don't pay much attention to what's coming in iteration N+1, much less iteration N+5. That means that every iteration brings with it a whole slew of what are, in effect, changed requirements. That trains both the software and the team to handle change as cheaply as is softwarically possible.

It's like the way that just-in-time inventory management forces factories to improve their production process. Because they cannot buffer asynchronies with stock on hand, they are forced to remove them. (See The Machine That Changed the World.)

## Posted at 23:53 in category /agile [permalink] [top]

The cost of change curve

Everyone knows the canonical cost of change curve (first image), where the cost of a change rises exponentially throughout the project. Let's pretend it's a law of nature, as many many project planners before us have done.

Now suppose someone notices they need to change a requirement in the middle of the project. The cost of change curve says that the change would be far too expensive. So it's not made.

Fine. But we know that almost every product that's actually used gets re-released with new features added. Most usually, the next project starts as soon as the previous one finishes. According to the cost of change curve, that's exactly at the point where the costs are highest (second image).

The decision to postpone changes can only make sense if the cost of change resets to something much closer to the far left of the curve (third image). What's supposed to make that happen? And why doesn't that operate in the middle of the first project's curve?

These are serious questions, even though I suspect I'm being stupid. I'm writing a book chapter that explains conventional and agile software development to an audience of sociologists, and this occurred to me. Mail me, and I'll summarize.

## Posted at 09:18 in category /misc [permalink] [top]

About Brian Marick
I consult mainly on Agile software development, with a special focus on how testing fits in.

Contact me here: marick@exampler.com.




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Working your way out of the automated GUI testing tarpit
  1. Three ways of writing the same test
  2. A test should deduce its setup path
  3. Convert the suite one failure at a time
  4. You should be able to get to any page in one step
  5. Extract fast tests about single pages
  6. Link checking without clicking on links
  7. Workflow tests remain GUI tests
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