estimating activity durations
DESCRIPTION
Presentation for project managers and team members on how to use software estimation techniques, based on well know practices.TRANSCRIPT
IKnowHow
Project Management Office
30 September 2013
Estimate Activity
Durations
Knowledge Area: Time Management
Who should do the estimation?
The estimators should be those who will be
doing the work, when possible.
The estimators are more often the members of
the project team who are most familiar with the
work to be done.
Padding
I have no idea how long it will take. I don’t even know what I am asked to do. So what do I say?
I will take my best guess and double it…
A pad is extra time or cost added to
an estimate because the estimator
does not have enough information
If your estimates are padded, you have
a schedule or budget that no one believes
The project manager’s role
Provide the team with information to properly
estimate each activity
Prevent padding
Complete a sanity check of the estimates
Make sure assumptions made during
estimating, are recorded for later review
The customer
If a customer want the shortest possible
schedule, they should not pressure you to
lower your estimates or to provide them with
misleadingly precise estimates.
Estimating activity techniques
I. One-Point Estimating
II. Expert judgment / Expert advice
III. Analogous Estimating (Top-down)
IV. Parametric Estimating
V. Three-Point Estimates
VI. Reserve Analysis
VII. Delphi & Wide-band Delphi Technique
One-point Estimating
The estimator submits one estimate per activity (…
that activity will take 5 days)
Maybe based on expert judgment, historical info or it
could be just a guess
Should only be used
for projects that do
not require a detailed
schedule
• Forces people to pad their estimates
• Hides important info about risks from the project
manager
• It can result in a schedule that no one believes
• Often results in the estimators working against the
project manager to protect themselves
• If an activity is estimated to 20d and is completed in
15d, it can make the estimator look untrustworthy.
Expert judgment
Expert judgment guided by historical information, can
provide estimates from prior similar projects and
experience
Vendors may be a good source
Analogous Estimating (Top-down)
Analogous estimating uses parameters such as
duration, budget, size, complexity from previous
similar projects, as the basis for estimating the same
parameters for a future project.
It is frequently used to estimate a project when there is
a limited information about the project (for example in
the early phases of a project).
It is generally less costly and time consuming than
other techniques, but is also less accurate.
Parametric Estimating
Uses statistical relationship between historical data
and other variables
For example, the estimator may use measures like time
per line of code
Three-point Estimates
The accuracy of activity duration estimates can be
improved by considering uncertainty and risk
This concept originated with the Program Evaluation
and Review Technique (PERT), which uses three
estimates to define an approximate range for an
activity’s duration.
PERT calculates a risk based Expected activity
duration using a weighted average of these three
estimates)
E = (o + 4m + p) / 6
where E is Estimate; o = optimistic estimate; p = pessimistic
estimate; m = most likely estimate
Reserve Analysis
Duration estimates may include contingency reserves
(time reserves or buffers) into the overall project
schedule to account for schedule uncertainty
May be a percentage of the estimated activity duration,
or a fixed number of work periods.
We should establish a reserve risk remaining after the
risk management process. THIS IS DIFFERENT THAN
PADDING
Delphi & Wide-band Delphi
Delphi is a group technique that extracts and
summarizes the knowledge of a group. The group are
experts who participate anonymously in order to
achieve consensus.
A request of information is sent to the experts, their
responses are compiled and the results are sent back
for further review until consensus is reached.
Usually uses three passes and the average of the third
guess is used as the groups estimate.
Wide-band Delphi combines Delphi and Three-
Point Estimating
Estimating Tips
Avoid off-the-cut estimates
Allow time for the estimate and plan it
Use data from previous projects
Use developer-based estimates
Estimate by walk-through
Estimate by categories
Estimate at a low level of detail
Don’t omit common tasks
Use several estimation techniques and compare
results
Rushedestimates are
inaccurate estimates
Sources:
A Guide to the Project management Body of Knowledge, 4th edition
(Project Management Institute)
Effective Software Project Management (2006, Robert Wysocki)
Rapid Development (1996, Steve McConnell)
PMP exam prep, 7th edition (Rita Mulcahy)