2+3 != 5¶

One of the common ways of estimating user stories is story points. The recommendation is use fibbonacci like sequence, and to assign numbers like 3,5,8 etc.

It's usually not explained why we cannot have 4 story points assigned to a story but ok, let's ignore it for now.

The thing is, story points are numbers, and numbers can be manipulated, added, subtracted, divided etc. And that's where it goes wrong. Before we know, the averages per sprint and team are calculated. The sprints are being compared, and story points converted to hours. Then it is used for forecasting, planning etc. etc.

But wait a second, are we sure that two stories of 2 SPs and 3 SPs together are the same as one story of 5 SPs? And combined with one more story of 3 SPs, do they cost as much as one story of 8?

If there is any reason to use fibonnaci, is to make sure that bigger stories get big faster. In my modest opinion it is just like this to reflect simple truth about software development, that it is easier, cheaper, faster to build in small increments (small stories) than in one big.

If so, then 5 means only "much bigger than 3", and not "2 + 3"... and just because of this, all kinds of aggregations of story points must be seen as very rough estimations, and not accurate calculations.

Since then, I believe it is much better to use t-shirt sizes like XS, S, M, L, XL for estimations, to prevent easy aggregations and calculations of things which are not meant to be aggregated.