In his brilliant post Don’t know what I want, but I know how to get it Jeff Patton used a sketch of Mona Lisa to illustrate the difference between Iterative and Incremental development.

Jeff points out that many Agile people use the word “Incremental” when they mean “Iterative”. I believe this is because, in reality, most Agile project combine both approaches and become Iterative Incremental. In fact I can’t imagine delivering software in any other way.

To illustrate what Iterative Incremental means I’ve taken Jeff’s Mona Lisa illustrations and added a third showing a combined Iterative Incremental approach.

Incremental

I’ll start with recapping Jeff’s explanation. On Incremental development Jeff says…

Incrementing calls for a fully formed idea
“incrementing” builds a bit at a time

Despite the fact “Incremental” is the term most often used by Aglists to describe what they do, the pure incremental approach is a very waterfall style. As Jeff points out the team knows the whole scope at the start – only possible with big up front analysis and design – then starts building out chunks. In the waterfall world this is called phased delivery.

Iterative

And for Iterative Jeff explains:

Iterating allows you to move from vague idea to realization
“iterating” builds a rough version, validates it, then slowly builds up quality

It is not an iteration if you only do it once

Notice here that the scope starts out quite vague and is fleshed out over time. But in this pure example the team starts to address the entire scope at the same time – which adds considerable risk in software delivery.

Iterative Incremental

Typically Agile teams combine Iterative and Increment approaches. An Agile team will conduct an Iteration (which happens to be XP speak for a Sprint) and produce a product Increment.

The Increment adds completely new features, based on user stories, hence expanding the scope of the functionality offered – that makes it Incremental. But each Increment is also likely to refine existing functionality – that makes it iterative.

The combined Iterative Incremental approach is much more powerful than either in isolation – at least in software development. It gives the product owner, team and company more choices. We can stop at any point and either throw away what we’ve done or launch it. This flexibility enables teams to build less but it also lets them innovate constantly.

Some examples based on the Iterative Incremental Mona Lisa. What do we get if we stop at each step … ?

• Step 1: We start the project but quickly realise it wasn’t a good idea and abandon it. We’ve lost the money it took to get that far but avoided paying the full cost of the project. Time to stop.
• Step 2: We have built the top priority functionality (Mona Lisa’s head) and we’re launching with that. Not sure we need the rest as the return on investment will be too low. Time to stop.
• Step 3: We’re now fleshed out the second tier of priorities (Mona Lisa’s hands and body). Not fully featured but filled in a few gaps that significant customers complained about. But we’ve realised the rest of the scope isn’t really needed. Time to stop.
• Step 4: We’ve got all the Must Have and Should Have items from the original high level requirements list (Mona Lisa’s head, hands and body). There are other things we could do but the business is starting up a new product line and needs to divert development effort to that. Time to stop.
• Step 5: We got all the major features and, in fact, we’ve touched on all the features from the product backlog. There is more we could do but really that would be more like gold plating than delivering essential functionality. Time to stop.
• Step 5: Wow. Impossible to improve. Time to stop.

It is during Agile planning that the team chops up the requirements into chunks (e.g. user stories) and the product owner articulates the priorities and decides what items to do when – both necessary for an Iterative Incremental approach.

Acknowledgement

The concept and illustrations are all Jeff Patton’s. I just copied, cropped and pasted to get the combined Iterative Incremental drawing.

Jeff wrote an excellent post and I recommend you read it if you haven’t already.

References

Patton, J. (2008, 1 January). Don’t know what I want, but I know how to get it. Agile Product Design.