Backlog estimates are adjusted when items are assigned to a Sprint.
This is because the initial estimate is made in a bit of a vacuum,
but the rubber hits the road when you assign it to a Sprint. Once you
know which Sprint, you know the staffing, location and other factors
that may force adjustments - things like: junior staff members, or
team that has not worked together before, etc.
This is covered in the Certification training materials
under "Activity: 1.4 Adjust Backlog Estimates", which includes an
example table of "drag" factors for different combinations of the
following: "# years team working together", "knowledge of
technology", "knowledge of domain". (Page 157 on my copy).
Note that this is just an example, and that different factors may
crop up where you work. The Adjustment activity is there to make the
Owner aware of mitigating factors *before* the fact. Alistair
Cockburn in Agile Software Development cites other factors such
as "non-colocated team". From experience, I'd also consider "slow
desktop CPU speed" as a significant drag factor when using continuous
integration with XP. (The appearance of this factor in your planning
could theoretically prompt a discussion between your Customer and
whoever approves IT hardware budgets... which could be good, right? :-
Here's what Activity: 1.4 says...
<snip - from Ken Schwaber's ScrumMaster Certif. materials 2003, p155>
To adjust the product backlog estimates to reflect any factors
reducing team effectiveness. Scrum assumes an optimal work
environment. This activity forces an understanding that suboptimal
product factors have a cost.
This activity is not mechanical, plugging in factors. Instead, it is
intended to provide cost awareness. Every decision regarding the team
working environment has an impact on their productivity and an
attendant cost. If the management is aware of this, this activity is
merely a reminder of these costs. If management is not aware of these
costs, this activity provides a monetary cost to suboptimal
Don't quibble about the factor values. They are generalized and not
intended to be precise for every situation.
--- In firstname.lastname@example.org
> I recently obtained the latest Scrum Excel add-in and user guide
> the Scrum Alliance web site. It is not clear to me when and how
> should update the product backlog estimates.