Definition of a Story Point
As far as I'm aware there's no definition of Story Points and how to compare them. Each person in a team may have his personal understanding of the correlation between an effort and Story Points. Isn't Story Points estimation just a fallacy?
Isn't it just a belief. For example, it's assumed that all tasks have a specific property - the difficulty, the amount of effort. But they maybe don't. And even if they do, it is just a belief that we can adequately estimate it as a number. The amount of time a task will take is intrinsically indeterminate.
For example: During Planning Poker all teammembers agree that a PBI should be estimated as 10 Story Points and they go to the next PBI. This 10 Story Point estimation actually means nothing because everybody understands 10 Story Points differently (different amount of effort, time, risks).
I just want reliable arguments (a research, comprehensive surveys) that SP is really a tool, and not just a belief.
Story points are a relative measure of effort rather than an absolute one. However, each member of the team should have the same understanding of the size of a points estimate. A common understanding is achieved when the team estimates repeatedly together and when they agree common baseline stories against which to measure. This is really no different to estimating in hours or days where people also measure things against remembered baselines. Planning poker is one way of making sure that teams have a common understanding of the size of items.
Relative estimation with story points has a few advantages over absolute estimation. It seems that many people come up with more accurate relative estimates than absolute ones. Velocity, as measured by story points completed per iteration, is an evidence-based measure whereas hours based estimates tend to be more subjective. If you measure things in hours then you can still retrospectively measure how many estimated "hours" you actually completed but that will inevitably differ from actual hours of work put in, so the reality is that "hours" tend to become a relative measure too.