... your Product Backlog Items need to be estimated.
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The teams’ estimates should be grounded in reality and not based on wrong assumptions or wishful thinking.
Sometimes when new work comes in, a caring manager may want to protect the team from the work of estimating it, and may take the initiative to estimate it themselves. It will take the team some energy to undo a number that was put in place, particularly if the number comes from someone in authority like a manager (with power over the team) or the Product Owner.
Development efforts tend to rely on experienced people and experts to do estimation. But maybe the people who are going to do the work are less experienced, or the team has forgot to involve an important area (as for example, test) in the estimates.
A team might not feel responsible for the backlog when others determine the estimates. If the expert is within the team, a hierarchy might emerge and the team as a whole will not take ownership of the estimates.
Let the people who are committed to do the actual work do the estimation. In the Scrum sense it is pigs that estimate — not chickens. , , 
The estimate should be a consensus generated from the perspectives of all relevant development areas. Research shows that estimates are much better when combining independent estimates, with iteration and feedback, from everyone who participates in development. Since Development Teams are cross-functional, it’s possible to get very good estimates created by the entire Development Team together. 
The terms Pigs (Developers) and Chickens (everyone else) finds its roots in the following joke:
A chicken and a pig are together when the chicken says, “Let’s start a restaurant!” The Pig thinks it over and says, “What would we call the restaurant?” The chicken says, “Ham n’ Eggs!” The pig says, “No thanks, I’d be committed, but you’d only be involved!” 
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The team feels committed to ownership for its work. This is good in its own right but will also increase the focus that the team brings to its estimation efforts. Oh, and one gets better estimates in the long term if they come from the Developers.
In particular, the Product Owner estimates the Business Value, and the Development Team estimates the cost or effort necessary to complete the PBIs. Each of these groups can use Wideband Delphi  or any other technique to build a consensus view (see Estimation Points). It is particularly important to capture the perspectives of all members of the cross-functional Development Team.
Estimates should continuously be revised as new information emerges. New Product Backlog Items can be estimated as the team is working on a Refined Product Backlog, and it is natural to review estimates at Sprint Planning. The Development Team can follow its intuition about which items should be re-estimated in light of new information. Also, as the Product Backlog is restructured in building a Refined Product Backlog or striving for a Granularity Gradient, the new items should be estimated in their own right.
In most applications of this pattern in Scrum, the estimate is only a forecast and should not be interpreted as a commitment. On the other hand, no one other than “the pigs” is allowed to speak to estimation. There should never be a ground for challenging an estimate à priori. For example, the Product Owner cannot impose his or her wishes for delivery on the team’s grounded forecast about how long work will take. Empirical insights from experience over time will help the team adjust its estimates to be more accurate.
 Ken Schwaber and Mike Beedle. Agile Software Development with Scrum (Series in Agile Software Development). London: Pearson, Oct 21, 2001, p. 35.
 Mike Cohn. Agile Estimating and Planning, 1 edition. Englewood Cliffs, NJ: Prentice-Hall, 2005, p. 51.
 Kenneth S. Rubin. Essential Scrum: A Practical Guide to the Most Popular Agile Process. Reading, MA: Addison-Wesley Signature Series (Cohn), Aug. 5, 2012, p. 123.
 Magne Jørgensen. “What we Know about Software Development Effort Estimation.” In IEEE Software 31(2), March/April 2014, pp. 37-40.
 Ken Schwaber and Mike Beedle. Agile Software Development with Scrum (Series in Agile Software Development). London: Pearson, Oct. 2001, p. 42.
 —. Wikipedia, “Wideband Delphi.” Wikipedia, https://en.wikipedia.org/wiki/Wideband_delphi (accessed 2 November 2017).