A user story containing unknown elements should be split into a spike to investigate the unknown elements plus a story to develop the functionality. This enables the customer to prioritize the research separate from the implementation of the new functionality.
Without a spike to substantiate the estimate for the user story, the customer may incorrectly assume the estimate to be valid and prioritize the story accordingly. Facing a spike and an associated story, the customer should prioritize the spike ahead of the story to obtain a more reliable estimate for prioritizing the story.
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