Last updated: 2022-03-18
Estimating overall project duration for larger projects is time consuming: when following the bottom-up approach of the project planning process,
we have to estimate the duration of each work package and then
integrate all data into a project schedule, usually, in form of a Gantt
chart which would only be available well into planning phase.
On the other hand, important stakeholders – like a customer or major
sub-contractors - ask us already in definition phase or early planning
phase for our estimated duration. Since the bottom-up approach is not
yet finished we start guess work about the project duration. Our
stakeholders take it seriously; they acquire our "guesstimation results"
as basis of their further preparation and planning. Later in planning
phase, we come back to them with our "real result", usually longer
durations, they realize that they have to change their plans and might
even become angry with us – very understandable.
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How can we resolve this dilemma? - Let us look at an example.
PROJECT DURATION IN A REAL LIFE EXAMPLE
slip into the role of a project manager, Adam, who works for
construction company YoNIWeBI (You Name It We Build It). In YoNIWeBI we
have a long history of managing all kinds of construction projects, one
of our divisions, AB, providing solutions of planning and constructing
large apartment buildings. Adam joined division AB 10 years ago and
managed apartment building projects ever since.
Division AB very much supports the idea of lessons learned workshops
at the end of each project, so that they have a large number of project records available, including information about duration of all kinds of work packages that are similar in scope.
two weeks ago, Adam took over a new project which is still in
definition phase. So far, it is clear that this new project is about
another apartment building with 20 floors and six 3-bedroom apartments
on each floor. Besides a meeting with major stakeholders, the customer
asks Adam for a quick estimate of the overall project duration, so that
they can kick off discussions with their financing partners. They would
like to know the probability of finishing the project within a certain
period of time.
So, Adam turns to AB's internal data base and finds the following, based upon records of projects of similar scope and volume:
Probability to complete definition, planning and design phase within
- 6 months: 10%
- 7 months: 50%
- 8 months: 40%
Probability to complete implementation and closure phase within
- 10 months: 20%
- 11 months: 40%
- 12 months: 40%
The question is: what is the probability for an overall duration of the project of 18 months or less?
APPLYING SIMPLE MATHEMATICS
remembers some math from high school and comes up with the idea of
applying the method of probability trees. Here is his solution:
Project Duration - Probability Tree
There are 6 different ways we can complete the whole project in 18 months or less, with respective probabilities:
- 6 months + 10 months, probability p = 10% * 20% = 2%
- 6 months + 11 months, probability p = 10% * 40% = 4%
- 6 months + 12 months, probability p = 10% * 40% = 4%
- 7 months + 10 months, probability p = 50% * 20% = 10%
- 7 months + 11 months, probability p = 50% * 40% = 20%
- 8 months + 10 months, probability p = 40% * 20% = 8%
By adding up the probabilities we obtain a probability of 48% for overall project duration of 18 months or less. Exactly this is Adam’s answer for the customer.
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