"Various projective practices upon trending have been used to forecast progress, like burndowns, burn-ups, or cumulative flows. These have proven useful. However, these do not replace the importance of empiricism. In complex environments, what will happen is unknown. Only what has happened may be used for forward-looking decision-making." -- Scrum Guide
Agile/Scrum teams are often asked to estimate how long a release might take. Or an entire project. Sometimes this is done under the guise of relative size estimates like T-shirt sizes – or, perhaps more commonly, story points – coupled with an estimated (or guessed) velocity. This is sometimes done even with new teams that have no velocity history.
Scrum, as defined in the Scrum Guide, places a large emphasis on the use of empiricism. Aside from the quote above, the following nuggets can also be found:
"Scrum is founded on empirical process control theory, or empiricism. Empiricism asserts that knowledge comes from experience and making decisions based on what is known. Scrum employs an iterative, incremental approach to optimize predictability and control risk."
"[The Scrum Master helps the Product Owner with] Understanding product planning in an empirical environment"
My interpretation of Scrum is that, while the Development Team are expected to estimate each PBI (Product Backlog Item), they are not asked nor expected to determine delivery dates, or how much work will be completed by a delivery date.
At Sprint Review:
"The Product Owner discusses the Product Backlog as it stands. He or she projects likely completion dates based on progress to date (if needed)"
So, the Product Owner uses the estimates on the PBIs combined with the empirical knowledge gained from what has actually been done to determine completion dates of a set of PBIs (e.g. a release). At no point does the Product Owner ask the team what will get done (beyond the current Sprint).
This use of empiricism is often neglected by Scrum teams. Teams are asked to project release dates, sometimes several months out, without any velocity history. This is not making projections based on what has actually happened. It is not empirical, and does not work in a complex, ever changing environment.
"A Product Backlog is never complete. The earliest development of it only lays out the initially known and best-understood requirements. The Product Backlog evolves as the product and the environment in which it will be used evolves. The Product Backlog is dynamic; it constantly changes to identify what the product needs to be appropriate, competitive, and useful. As long as a product exists, its Product Backlog also exists."
If you are using estimates, it is important that you use probabilistic estimates based on real, empirical data. Scrum suggests this. Practitioners suggest this also. Don’t ask the team to forecast any further out than the current Sprint. As the Product Owner, use real data to make forecasts and decisions. Asking the team to make longer term projections is not respecting the data showing what is actually getting done.
Monitor progress, don’t try and predict it.