The basis of the estimating process is “Team based relative size estimates” or “Story Point estimation” to estimate User Stories (the need). This means:
If two work items are the same size, should they always have the same estimate?
It is worth delving into the notion of a relative size estimate for a moment. Most traditional estimates are based on duration (“it will take about 20 hours), and that leads to a whole bunch of problems (see What Are The Problems with Estimation? for more information). But it is worse than that. If you have two pieces of work to do, both of which you think will take about 20 hours to complete, but the first piece of work is straight forward, while the second is in an area where there is a traditionally a lot of problems, should these items have the same estimate? No, you'll probably want to put some level of buffer in place to offer a more realistic estimate for the second piece of work. This is another reason to move away from pure, absolute time based estimates.
One thing that often confuses people who are used to traditional task based estimates is that the resultant estimate is a Team's view of how big this item is. When we say the estimate is a “3”, we are really saying that the Team's view of this piece of work is that it is a “3”. In particular, it is not a single Team member's view. When starting, many Teams fall into the trap of saying “There are design, implementation, and testing components of this piece of work. George will do the design so what do you think the estimate is for the design piece … Jenny will implement, so what do you think the estimate for the implementation piece is …” and then just sum up the results. In Agile the unit of execution is the “Team” and so the estimates indicate the size of work for the Team not the individuals on the Team. Sure the time to “design” and “implement” are part of that. But to deliver the value represented by the Story the Team might, for example, want to pair implementation and testing as they do the design to improve the design. Or the Team might find that testing might need an “all hands on deck” approach to assure quality. What we are estimating is the Team's ability to deliver value.
In Agile the unit of execution is the “Team” and so the estimates indicate the size of work for the Team not the individuals on the Team.