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by Martin Fowler.
Original Post: Bliki: PurposeOfEstimation
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My first encounter with agile software development was working
with Kent Beck at the dawn of Extreme Programming. One of the things
that impressed me about that project was the way we went about
planning. This included an approach to estimating which was both
lightweight yet more effective than what I'd seen before. Over a
decade has now passed, and now there is an argument amongst
experienced agilsts about whether estimation is worth doing at all,
or indeed is actively harmful . I think that to answer this question
we have to look to what purpose the estimates will be used for.
A common scenario runs like this:
Developers are asked for (or given) estimates for upcoming work.
People are optimists, so these estimates tend to be too low,
even without pressure to make them low (and there's usually at
least some implicit pressure)
These tasks and estimates are turned into release plans
tracked with burn-down charts
Time and effort goes into monitoring progress against these
plans. Everyone is upset when actuals end up being more
than estimates. In effort to
increase pace with the estimates, developers are told to sacrifice
quality, which only makes things worse.
In this narrative, effort put into estimates is, at best, waste
- since "an estimate is a guess in a clean shirt" . Usually estimates end up being actively harmful as
they encourage FeatureDevotion, a nasty condition where
people start valuing ticking off features more than tracking the real
outcome of the project.
Estimates also set expectations, and since estimates are usually
too low, they set unrealistic expectations.  Any increase in time or
reduction in features is then seen as a loss. Due to loss-aversion,
these losses have a magnified effect. 
Faced with situations like this, it's easy to see how people
turn their angry glares towards estimation. This leads to an
increasing notion that anyone indulging in estimating is an Not a
True Agilist. Critics of agile say this means
that agile development is about developers going off and doing vague
stuff with promises that it'll be done when its done and you'll like it.
I don't share this view of estimation as an inherently evil
activity. If I'm asked if estimation is a Bad Thing my answer is
the standard consultants' answer of "it depends". Whenever someone
answers "it depends" the follow-up question is "upon what". To
answer that we have to ask why we are doing estimation - as I like
to say "if it's worth doing well, it's worth asking why on earth you're doing
it at all".
For me, estimation is valuable when it helps you make
a significant decision.
My first example of an estimation-informed decision is allocation
of resources. Organizations have a mostly fixed amount of money and
people, and usually there are too many worthwhile things to do. So
people are faced with decisions: do we do A or B? Faced with such a
decision it's useful to know how much effort (and cost) each will
involve. To make sensible decisions about what to do, you need to
have a feel for both the cost and the benefits.
Another example is to help with coordination. The blue team
wants to release a new feature to their web site, but cannot do so
until the green team builds a new service to give them crucial
data. If the green team estimates they will be done in two months
and the blue team estimates that it will take them a month to build
the feature, then the blue team knows it's not worthwhile to start
today. They can spend at least a month working on some other
feature that can be released earlier.
So whenever you're thinking of asking for an estimate, you
should always clarify what decision that estimate is informing. If
you can't find one, or the decision isn't very significant, then
that's a signal that an estimate is wasteful. When you do find a
decision then knowing it focuses the estimate because the decision
provides context. It should also clarify the desired precision and
Understanding the decision may also lead you to alternative
actions that may not involve an estimate. Maybe task A is so much
more important than B that you don't need an estimate to put all
your available energies into doing it first. Perhaps there is a way
for blue team members to work with the green team to get the
service built more quickly.
Similarly, tracking against a plan should also be driven by how
it informs decision making. My usual comment here is that a plan
acts as a baseline to help assess changes - if we want to add a new
feature, how do we fit it into the FivePoundBag?
Estimates can help us understand these trade-offs and thus decide
how to respond to change. On a larger scale re-estimating a whole
release can help us understand if the project as a whole is still
the best use of our energy. A few years ago we had a year-long project
that was cancelled after a re-estimate a couple of months in. We
saw that as a success because the re-estimate suggested the project
would take much longer than we had initially expected - early
cancellation allowed the client to move resources to a better target.
But remember with tracking against plans that estimates have a
limited shelf life. I once remember a gnarly project manager say
that plans and estmates were like a lettuce, good for a couple of
days, rather wilty after a week, and unrecognizable after a couple
Many teams find that estimation provides a useful forcing
function to get team members to talk to each other. Estimation
meetings can help get better understanding of various ways to
implement upcoming stories, future architectural directions, and
design problems in the code base. In this case any output
estimation numbers may be unimportant. There are many ways such
conversations can happen, but estimation discussions can be
introduced if these kinds of conversations aren't happening. 
Conversely if you're thinking of stopping estimation, you need to
ensure that any useful conversation during estimation still
Go to any conference with agile leanings and you'll hear talks
of teams that work effectively without estimation. Often this works
because they, and their customers, understand that making estimates
isn't going to affect significant decisions. An example is a small
team working closely with business. If the broader business is
happy with allocating some people to that business unit, then work
can be carried out in priority order; often this is helped by the
team breaking down work into small enough units.  A team's level in the agile fluency
model plays a big role here. As teams progress they first
struggle with estimation, then can get quite good at it, and then
reach a point where they often don't need it. 
Estimation is neither good or bad. If you can work effectively
without estimation, then go ahead and do without it. If you think
you need some estimates, then make sure you understand their role
in decision making. If they are going to affect significant
decisions then go ahead and make the best estimates you can. Above
all be wary of anyone who tells you they are always needed, or
never needed. Any arguments about use of estimation always defer to
the agile principle that you should decide what are the right techniques for
your particular context.
A recent read is Estimation
is Evil an excellent discussion by Ron Jeffries of the
problems that estimates can cause.
I got this analogy from Ron Jeffries, although I don't have a
written reference for it.
I particularly liked a comment on this by my colleague Angela
Ferguson "the way that estimates set expectations is up to us -
it is poor project management (whether by project managers or
other team members) that results in a client who thinks
estimates are fixed, or that raw estimates = actual
"I, in fact, try to practice delivery bad news on a weekly
basis with my key client, even when things are travelling as
expected ... 'so we're looking quite well on track now, but if
we had discovered something that took longer than expected, or a
requirement had blown up to be larger than expected, or we found
something new and very important, what do you think the best
course of action would be?' And then you explore the options -
cut stories, add time, add capacity, etc. This means that when
the expected unexpected thing happens (because we know it will
happen), the conversation doesn't seem new and scary to the
Very roughly people feel twice as much pain for a loss as
pleasure for a gain.
If you do this an approach like ThrownEstimates can
help the discussion move at a good pace.
Of course breaking work down into small units requires some
implicit estimation, but that's really a different animal to
the more common explicit estimation activity.
James Shore has a recent blog post that details his observations
about how fluency influences estimation practice. I think a
similar analysis of practices at various stages of fluency could
be very useful.
I repeat myself again in thanking varous ThoughtWorkers on internal
lists for their comments. In particuarly I'd like to call out Angela
Ferguson, Dave Pattinson, and Pat Kua. I should also thank James Shore for responding
so rapidly and well for my question about the links to the fluency model.