Large effort estimation
1. Structure your backlog across levels like in a user story map
Nigel Thurlow and Dave Slayton came up with this approach to estimate scope, at Toyota. You can use this to estimate size at the start of a project. You can also use this technique when you have to estimate a large amount of scope for an in-flight project. The technique works very well in conjunction with Jeff Patton’s story mapping technique. No worries if you aren’t familiar with that. You need to structure the product backlog from your inception at three different levels.
Themes - these can be categories, features, or system capabilities.
Epics - these are large stories that are just too big to deliver in a single sprint.
Stories and tasks - these are fine grained items that you can deliver within a sprint.
2. Size your themes and epics independently from one another
Once you have your three-tier backlog in place, you need to relatively size the themes. Let’s imagine a backlog with three themes, which your developers have sized at 5, 8 and 3 respectively. Next you need to independently size the epics under any one of the epics. By “independently”, I mean that the sizes at a level have nothing to do with the level above or below it. Let’s take the theme sized “3” and assume that the epics under it have the sizes 1,3 and 5. Your estimated story map should look a bit like the above diagram now.
3. Estimate the stories under one epic
Next, pick any of the epics and independently size the stories and tasks underneath it. In the interest of simplicity let’s take the epic sized “1” in our example. Let’s imagine that the sum estimates for stories underneath that epic is 8 points. This is where you can start extrapolating. Let’s estimate the entire backlog for the example we’re discussing.
3 theme points = 9 epic points
Therefore, 1 theme point = 3 epic points
1 epic point = 8 story points
Therefore 1 theme point = 3 epic points = (3 x 8) story points = 24 story points
Our entire backlog has three themes.
The sum of theme points = 5 + 8 + 3 = 16
Therefore, the size of the entire backlog (24 story points per theme point) = 16 x 24 = 384 story points
And there you have it. By diving deep into one epic from your story map you can extrapolate your estimates to the entire backlog. Nigel says that you don’t need to do more than one chunk. It doesn’t make you any more accurate.
“We found that just taking one chunk, breaking that all the way down to user stories and then being able to extrapolate that to size; was around 90% accurate, we couldn't get it to be any worse than that. We thought, well, if it was 70% accurate, we'd be happy. But in the end, it turned out we were incredibly accurate.” - Nigel Thurlow
Both this method and warp-speed estimation, are incredibly fast to execute. They make for efficient, synchronous workshops to size your initial backlog and get a sense of your raw scope.