Backlog Refinement, also called Backlog Grooming in some circles, is the process of refining a user story in preparation for sprint planning which includes estimation of the effort required to complete user stories. During backlog refinement acceptance criteria for the story are defined and a sketch of the solution design is specified. Both of those items need to be detailed in a user story for you to be able to estimate the effort.
Software development work is notoriously difficult to estimate. Agile methods recommend relative estimation techniques such as Planning Poker. After a story has been analyzed and designed it can be estimated. The team must come to a consensus on the estimate or discuss and vote again until consensus is reached.
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