Some Scrum practitioners estimate the effort of product backlog items in ideal engineering days, but others prefer less concrete backlog effort estimation units. Alternative units might include story points, function points, or “t-shirt sizes” (1 for small, 2 for medium, etc). The advantage of more vague units is that they’re explicit about the distinction that product backlog item effort estimates are estimates of effort, not duration. Also, estimates at this level are rough guesses that should never be confused with actual working hours (Note that sprint tasks are distinct from product backlog items and task effort remaining is always estimated in hours).