Although all of these approaches are valid, some will work
better than others. The best approach will depend upon factors
such as the availability of historical data, the estimating skills of
task performers or subject matter experts, and the amount of
time available to prepare an estimate. You may want to try
more than one approach, then use your judgment to come up
with the best estimate. Remember: estimates should reflect
what you believe to be the most likely outcome. Don’t be afraid
to apply your own judgment to the input you receive, as long as
you have a rational reason to do so.
Estimating Pitfalls
Estimating is difficult. There are many things that can under-
mine the accuracy or validity of your estimates. Among the
most common pitfalls are the following:
- Poorly defined scope of work. This can occur when the
work is not broken down far enough or individual ele-
ments of work are misinterpreted. - Omissions.Simply put, you forget something.
- Rampant optimism. This is the rose-colored glasses syn-
drome described previously, when the all-success sce-
nario is used as the basis for the estimate. - Padding.This is when the estimator (in this case almost
always the task performer) includes a factor of safety
without your knowledge,a cushion that ensures that he
or she will meet or beat the estimate. - Failure to assess risk and uncertainty.As mentioned ear-
lier, neglecting or ignoring risk and uncertainty can result
in estimates that are unrealistic. - Time pressure.If someone comes up to you and says,
“Give me a ballpark figure by the end of the day” and
“Don’t worry, I won’t hold you to it,” look out! This almost
always spells trouble. - The task performer and the estimator are at two differ-
ent skill levels. Since people work at different levels of
efficiency, sometimes affecting time and cost for a task
An Overview of Planning and Estimating 109