Software is not like a house

“Building a house” is perhaps the most overused software analogy out there. Sure, there are many overlaps and it’s a concept that non-technical people can grasp easily when we need to explain something, but it simply doesn’t add up. I’ve ran into this analogy frequently in my current obsession with estimation (someone even used it to comment on one of my recent articles) so I’ve been compelled to take this on as well ;-). Below is is a typical example of an argument I hear all the time. It was posted in response to this article on InfoQ:

“So, you’re going to remodel your old house. It’s a two-story Victorian, and you want to knock out half of the back exterior wall, expand the kitchen, and a 3/4 bathroom, and put a new bedroom above all that on the second floor.

You call up a contractor, and she says that she won’t give you any estimates, but she will show clear progress on a weekly basis, and you’re free to kill the contract at the end of any given week if you’re not satisfied.

So you begin work. They blast out the back wall, frame up the new rooms, and they’re beginning work on the kitchen expansion, when you begin to realize that you’re burning through your savings faster than you expected, and you’re not sure if you’ll be able to complete the job as you’d like. In fact, you’re beginning to worry if you’ll be able to get the thing completed in any useful way at all. At this point, you ask the contractor for an estimate of the work remaining to be done, and she gleefully responds, ‘I don’t give estimates, but you can cancel at any time!'”

If a house was like software and I was a contractor given the above conditions this is what I would do: I would build something that was usable and fulfilled the absolute minimum requirements as soon as possible (e.g. a wooden shed extension with a camping stove), see what the customer thinks and then rebuild it, say, every two weeks adding more and more of the requirements and responding to their feedback as we go, making sure that at the end of every two weeks the customer still had something they could use. In the end we’d have fulfilled as many of the requirements possible given the allocated money and time frame. We wouldn’t have gone over budget, been able to respond to unexpected events (e.g. finding out the foundations are not sufficient) and the customer could change his mind (on windows, room layouts etc) as we go.

Crucially, what the author of the above comment fails to grasp is the difference between incremental and iterative development (see end of article). Developing software (notice how no one says “building” software) is not like building a house – we have the luxury that it’s relatively inexpensive (if we’ve followed good design principles) to go back and change it as often as we like. I say, tell me honestly how much you’ve got to spend/how much time you have and I’ll give you the best possible value for those conditions. If an unexpected event occurred which means we can’t deliver it, well, it would have happened if we’d estimated it anyway. At least my way (well, it’s not my way is it…) we’re more likely to find out sooner and hopefully save the customer some money/embarrassment.

If you’re going to argue with me (which I dearly hope you will) please do not use this tired analogy. At least now rather than engaging with you I can point you this way and I don’t have to repeat myself 🙂

Further reading:

Jeff Patton explains on his blog the important difference between iterative and incremental development. – an exert from an excellent presentation I saw him give at XPDay 2007
The Software Construction Analogy is Broken. A very thorough article posted on kuro5hin a while back.

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  1. Pingback: rob bowley - adventures in software craftsmanship » Blog Archive » best of Apr ‘08 to Apr ‘09

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