Breaking large user stories (or work items in layman’s terms) down into several smaller ones hugely improves delivery predictability. There are several reasons for this including the fact that smaller items with fewer moving parts are simply easier to estimate for humans. There are fewer problems to solve, fewer assumptions to get wrong and fewer things to overlook. it’s common to hear developers saying things can’t be broken down further, however this is rarely the case.
Breaking things down does require practice and a little creativity to ensure each item still provides business value but is independent enough to be moved up or down the list of priorities when adapting to business needs. Tony Heap provided a great list of tips and techniques for breaking down user stories in his session at Agile Yorkshire. This list isn’t exhaustive but cover many of the most common situations.