Qfyilyi

I've read in several places that story points are not necessarily linear.

i.e., an "8 point" task is not the same as two 4 point tasks and so on.

I totally get the argument about these being an indication of complexity rather than time taken.

But if they're not a linear scale, then how can you do arithmetic on them? If an 8 story point takes, say, 3 times longer than 2 x 4 story points, then how do burndown charts work from an arithmetic point of view?

If our velocity is, say, 30 a sprint then this means we could do 30 x 1 story point features. But these might be, 30 half an hour jobs. Equally if it was 2 x 15 story point features, these are probably monster tasks which seems equally unlikely.

Perhaps I am wrong in my assertion that they are non-linear?

Or can anyone explain this to me?

Thanks!

I have never heard of this. Story points are linear (otherwise it would be impossible to use them as a measure of velocity). However the scale is non-linear, to stop people arguing over whether something is a "5 or a 6" - by using a psuedo-fibonacci sequence, you automatically account for the vagueness of estimation.

Perhaps a more accurate way to put it would be that story point estimates are imprecise. If you have a 5 and a 3, that may or may not be the same size as an 8.

To make this less confusing, let's start with a non-numeric scale like T-Shirt sizes. XS, S, M, L, XL and so on. We can agree pretty easily that a small and a medium t-shirt do not get you a large t-shirt. Yet, a large is bigger than a medium and a lot bigger than a small, and generally smaller than an XL. Not always, of course. We all know that one company that we have to buy a different size in. User stories are the same way. It's possible I have a M that is actually bigger than some L, but this is the exception, so I can normally assume that a L is one step bigger than an M.

OK, now let's do this: XS-1, S-3, M-5, L-8, XL-13. Now, all of the same rules apply. It is possible in some edge cases that a 5 is actually bigger than some 8, but generally speaking an 8 is one step bigger than a 5.

Then there is the topic of velocity. Because the relationship between the sizes is generally consistent, we can add the sizes together and if we work at a consistent pace we will have a fairly consistent total. It won't be perfect - maybe 45 - 52, but that is consistent enough to be useful for planning. If you have 35 points in the sprint, it is probably too little in this case and 60 is almost certainly too much. This is also why most forecasts are a range, not a precise measurement.