Project Initiation, Planning and Execution
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MBA642_T3_2020_Workshop_03.pdf
MBA642
Project Initiation,
Planning and Execution
Workshop Week 3
Project Scheduling: Duration
Estimation and Critical Path
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Week 2 Review
• Scheduling
• Network Diagrams
• Activity Networks
• Activity on Node
• Perform activity duration estimation based on the use of probabilistic estimating techniques
• Construct the critical path for a project schedule network using forward and backward passes
• Identify activity float and the manner in which it is determined
• Understand the steps that can be employed to reduce the critical path
Week 3 Lecture Objectives
Workshop Activity In groups of 3 or 4, using your smartphone or tablet, do
a search on the Internet of the term “project
scheduling”. Thousands of hits will be generated.
Examine a selection of these results and discuss with
the class what common themes you found. Take the
best features of these and write your own definition.
Explain why it is the best version.
Duration Estimation – How?
Two primary means for developing duration estimates:
1. Deterministic models for activity durations where project
tasks are fairly predictable (i.e. little variation in the activity
completion time).
2. Mathematical models for where tasks are unpredictable,
we may use to predict likely duration.
Activity Durations – How?
Experience
– If similar work, we can use past experience as a guide. This approach is
relatively easy
– Main weakness is that it assumes what worked in the past will be
relevant today.
– We must also be aware of the potential for using distorted or outdated
information.
Activity Durations – How?
Expert opinion
– Intuitively this approach would seem to be useful.
– Potential weakness: an expert’s estimate of completion time may not
necessarily be valid for non-experts doing the same activity.
Mathematical derivation
– Benefit: offers a more objective approach to activity duration estimation
and avoids many of the problems that can be found in more subjective
methods.
– This method involves developing duration probability based on a logical
analysis of best-case, most likely case, and worst-case scenarios.
Workshop Activity
On your smart phone or tablet, use a search engine to
find “how long does it take to build a house?”
In groups of 3 or 4, discuss your findings. What are
some of the variables that impact on the time taken to
build a house?
How might you go about planning for such a project?
Duration Estimation
To derive a reasonable probabilistic estimate for
an activity’s duration, we need to identify three
values:
1. The activity’s most likely duration
2. The activity’s most pessimistic
duration
3. The activity’s most optimistic duration
Workshop Activity The following terms are commonly used in probability
statistics when applied to Duration Estimation for projects:
• Mean
• Median
• Mode
• Standard deviation
• Normal distribution
• Standard deviation
• Variance
• Confidence interval
Divide into groups of 3 or 4 and select 2 terms and explain
them to the rest of the class using simple examples. You can
use internet searching for this task if that helps.
Duration Estimation Using Probability
• Probability distributions can either be:
- Symmetrical (normal distribution) or
- Asymmetrical (beta distribution)
• Normal distribution – implies that the probability
of an event taking the most likely time is one that
is centered on the mean (average) of the
distribution.
• Pessimistic and optimistic values – estimated
at the 95% CI from either end of the distribution,
so cancel each other out.
• Hence the mean value is the expected duration
time for the activity.
Duration Estimation Using Probability
Source: Pinto, JK 2016, Project Management, Achieving Competitive Advantage, 4th Edition, Pearson Education Inc., p329
Symmetrical (Normal) Distribution
Duration Estimation Using Probability
Source: Pinto, JK 2016, Project Management, Achieving Competitive Advantage, 4th Edition, Pearson Education Inc., p329
Asymmetrical (Beta) Distribution for Activity Duration estimation
Duration Estimation Using Probability
Source: Pinto, JK 2016, Project Management, Achieving Competitive Advantage, 4th Edition, Pearson Education Inc., p329
Two assumptions for converting the values of m, a, and b into
estimates of the expected time (TE) and variance (s2) of the duration
for the activity:
1. The standard deviation (a measure of how spread out your data is)
of the duration required to complete the task (s) equals one-sixth of
the range for reasonably possible time requirements.
2. Because the probability distribution are not symmetrical about the
mean, beta represents the distribution of possible alternative
expected duration times (TE) for estimating activities.
Duration Estimation Using Probability
Source: Pinto, JK 2016, Project Management, Achieving Competitive Advantage, 4th Edition, Pearson Education Inc., p329
The beta distribution suggests that the calculation for deriving TE is
shown as:
TE = (a + 4m + b) / 6
where
TE = estimated time for activity
a = most optimistic time to complete the activity
m = most likely time to complete the activity (mode of the
distribution). The mode of a set of data values, is the value that appears
most often.
b = most pessimistic time to complete the activity
Duration Estimation Using Probability
Source: Pinto, JK 2016, Project Management, Achieving Competitive Advantage, 4th Edition, Pearson Education Inc., p329
Duration Estimation Using Probability
Source: Pinto, JK 2016, Project Management, Achieving Competitive Advantage, 4th Edition, Pearson Education Inc., p329
Workshop Activity
Duration Estimation
Activity Optimistic Likely Pessimistic Beta (1:4:1)/6
Planning 4 6 8
Foundations 2 3 7
Framing 3 5 10
Windows and Doors 1 2 5
Brickwork 3 5 8
Roof covering 2 3 6
Flooring 1 2 4
Electrical and
Plumbing
1 2 5
Painting 3 5 7
Constructing the Critical Path
• Critical path calculations link activity
durations to the pre-constructed project
activity network.
• Project network – we’ve developed using
activity precedence logic.
• Next, following task duration estimates, the
values are applied in a structured process to
each activity to determine overall project
length.
Constructing the Critical Path
Source: Pinto, JK 2016, Project Management, Achieving Competitive Advantage, 4th Edition, Pearson Education Inc., p331
Constructing the Critical Path
Source: Pinto, JK 2016, Project Management, Achieving Competitive Advantage, 4th Edition, Pearson
Education Inc., p332
Partial Project Activity Network with task Durations
Four separate paths can be identified, labelled as follows:
Path One: A - B - E - H
Path Two: A - B - D - F - H
Path Three: A - C - D - F - H
Path Four: A - C - G - H
Constructing the Critical Path
Critical path (definition) – “series of interdependent
activities of a project, connected end-to-end, which
determines the shortest total length of the project.”
In the previous example; the shortest total length of
time needed to complete a project is determined by
the longest path through the network.
The length of the four paths listed above can be
derived simply by adding their individual activity
durations together.
Constructing the Critical Path
The length of the four paths can be obtained simply by
adding their individual activity durations together.
Hence,
Path One: A - B - E - H : 18 weeks
Path Two: A - B - D - F - H : 29 weeks
Path Three: A - C - D - F - H : 30 weeks
Path Four: A - C - G – H : 22 weeks
Path Three is the critical path for this activity.
Workshop Activity In groups of 3 or 4, using your smartphone or
tablet to find definitions for “forward pass” and
“backward pass”.
Why are these concepts important in Project
Management? Share your thoughts with the
class.
The Forward Pass
The forward pass – used to
determine the earliest times
each activity can begin and the
earliest it can be completed.
A. Starts day 0, takes 5 days,
so finishes day 5.
B. Starts after A finishes on
day 5, takes 5 days, so
finishes day 10 (5+5).
Partial Activity Network with Merge Point at Activity D
Source: Pinto, JK 2016, Project Management, Achieving Competitive Advantage, 4th Edition, Pearson Education Inc., p333
The Forward Pass
Activity Network with Forward Pass
Source: Pinto, JK 2016, Project Management, Achieving Competitive Advantage, 4th Edition, Pearson Education Inc., p334
The Backward Pass
Source: Pinto, JK 2016, Project Management, Achieving Competitive Advantage, 4th Edition, Pearson Education Inc., p334
Activity Network with Backward Pass
The Backward Pass
Source: Pinto, JK 2016, Project Management, Achieving Competitive Advantage, 4th Edition, Pearson Education Inc., p335
Project Network with Activity Slack and Critical Path
Note: Critical path is indicated with bold arrows.
The Backward Pass
Source: Pinto, JK 2016, Project Management, Achieving Competitive Advantage, 4th Edition, Pearson Education Inc., p336
• Note that activity float is determined as a result of
performing the forward and backward passes
through the network.
• Until we have done the calculations for ES, EF, LS,
and LF, we cannot be certain which activities have
float associated with them and which do not.
• Using this information to determine the project critical
path suggests that the critical path is the network
path with no activity slack associated with it. In our
project, we can determine the critical path by linking
the nodes with no float: A - C - D - F - H.
The Backward Pass
Source: Pinto, JK 2016, Project Management, Achieving Competitive Advantage, 4th Edition, Pearson
Education Inc., p336
• We can also determine path float; that is, the linkage of each
node within a noncritical path.
• The path A - B - E - H has a total of 13 weeks of float; however,
it may be impossible to “borrow” against the float of later
activities if the result is to conflict with the critical path.
• Although there are 13 weeks of float for the path, activity B
cannot consume more than one week of the total float before
becoming part of the critical path.
• This is because B is a predecessor activity to activity D, which
is on the critical path.
• Using more than one week of extra float time to complete
activity B will result in delaying the ES for critical activity D and
thereby lengthening the project’s critical path.
Workshop Activity
In groups of 3 or 4, go to the following website:
https://theconstructor.org/construction/critical-path-method-cpm-
advantages/6873/
Select one of the stated advantages of CPM (Critical Path Method)
and be prepared to critique it with the rest of the class.
Steps to Reduce the Critical Path
Source: Pinto, JK 2016, Project Management, Achieving Competitive Advantage, 4th Edition, Pearson Education Inc., p340
Often when constructing an activity network and seeing the
expected duration, we may look for ways to shorten the
project. The list below shows some of the more common
methods for reducing the critical path.
Summary
• Understand the practical application of scheduling
terminology.
• Apply the logic to create activity networks.
• Develop an activity network using Activity-on-Node
techniques.
• Perform activity duration estimation based on the
use of probability estimating techniques.
• Construct the critical path for a project schedule
network using forward and backward passes.
• Identify activity float and how it is determined.
• Apply lag relationships to project activities
• Construct and comprehend Time & Action
(Gantt) charts
• Recognise alternative means to accelerate
projects, including their benefits and drawbacks
• Understand the trade-offs required in the decision
to crash project activities
Next Week
