... you are organizing the work of the enterprise, and the Product Owner is looking for ways to get help from the team in preparing requirements and the Product Backlog.
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All work needs to be accounted for if the Product Owner is to use team velocity for release planning.
The Product Backlog is the primary tool for budgeting the work on a project. Its Product Backlog Items (PBIs) are ordered by the Product Owner, who takes development estimates into consideration when planning deliveries. Teams are expected to eventually commit to their estimates.
However, there are some kinds of work which are difficult, or impossible, to estimate. Some analysis tasks require open-ended research to evaluate development feasibility, cost, and tradeoffs. You can time-box such research, but experience shows that that results in repeated extensions of the research time. And while research activities produce insights, they rarely produce the kind of potentially shippable increment that is mandatory for items taken into a sprint. This raises doubts about doing such work within the Sprint, and about budgeting it on the Product Backlog.
This wouldn't be a problem if the Product Owner alone could do the research, because the Product Owner can manage his or her own time. However, it is often the case that Product Owners have a strong business skill set but a weaker skill set to actually build the artifacts to be evaluated.
Scrum tradition has many other activities whose work is not tracked on the Product Backlog. Examples include sprint planning, drag, sprint review, and product backlog grooming.
Compartmentalize the planning work for long-running high-risk business items into periodic meetings that are outside the Sprint budget.
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Examples include the sprint planning meeting and product backlog refinement meeting. However, this idea might be generalized to include analysis Spikes by the team, particularly if the Spikes would otherwise of uncertain length.
The Product Owner can still fund the team to work on well-contained PBIs within the sprint. For example, if the Product Owner needs a prototyping tool and can deliver an Enabling Specification that describes what is needed, it is fine to use a PBI to direct the team to do this work. Such a tool feeds the value stream and has demonstrable return-on-investment.