7/27/06
MODULE 4: SCHEDULING, BUDGETING AND RESOURCES
1. Finish Scheduling using CPM and Gantt Charts
2. Estimating resources
3. Budgeting
1. Scheduling – Part 2
Scheduling for us means to fit the work between fixed start and finish dates. It is
made of tasks or activities that require resources. Resources and scope of work fix the
duration of the activities. Failure to consider resources almost always leads to a
schedule that cannot be met.
In working out the first version of the schedule, assume unlimited resources. This is
important, because it will show what kind of resources are needed to make the
schedule work. In the second version, resources have to be factored in and the
schedule made more realistic.
Here is how scheduling is done.
I. Start with a CPM Diagram
CPM stands for Critical Path Method. The origin of this name will become clear
shortly.
Step 1: From the WBS, put the activities leading to the goal of the project in a logical
chronological order.
This is represented in Step 1 of the Yard Work CPM Diagram. At this step, all that
matters is that activities be connected in a logical chronological order together with
an estimate of the time it takes to complete each activity, represented by DU (stands
for duration). There is usually not a unique way to produce this diagram.
Each activity/task box has the following symbols:
ES stands for Early Start
LS stands for Late Start
EF stands for Early Finish
LF stands for Late Finish
These terms are explained below, using the yard work example from the
“Fundamentals of Project Management” book.
Step 2: Estimate and post the ES and EF times
These are estimated moving forward in time through the diagram. Start the project at
time zero. In the yard work example, there are three activities that can be done in
parallel (ignoring for the time being that this requires at least three people who can
work independently). The three parallel activities, “Pick-Up Trash”, “Put Gas in
Equipment”, and “Get Hedge Clipper” take different times but all start at time zero.
“Trimming the Hedges” is an activity that is independent of the “Pick-Up Trash”, and
“Put Gas in Equipment” activities and whose ES time depends only on the EF time of
“Get Hedge Clipper” activity. “Pick-Up Trash” takes longer than “Put Gas in
Equipment” and determines the ES times for “Trim Weeds”, Mow Front Yard, and
“Edge Sidewalk”. No work can start before all the trash has been picked up, except
for “Trimming the Hedges”. The ES for “Mowing the Back Yard” is the EF for
“Mowing the Front Yard” as there is only one lawn mower. Had there been two lawn
mowers, both activities could be done in parallel. This is an example of resources
affecting the schedule. Depending on conditions, “Trimming Hedges” could interfere
with mowing the lawns and may have to be postponed. Among parallel activities the
longest is mowing the backyard. Thus the EF of “Mowing Backyard” is the ES for the
next phase: bagging the grass and bundling the trash. Again bagging the grass takes
longer than bundling the trash and thus its EF imposes the ES for the final activity
“Hauling the Trash”.
Step 3: Estimate and post the LF and LS times
We’ll see why this is important in a moment.
The LF and LS times are estimated moving backward in time starting with the last
activity and using the EF for the final activity as a starting point: 165 minutes. To do
this we will always us the smallest number for the LF of previous activities.
Starting with “Hauling the Trash” which has an LF=EF = 165 minutes, we subtract
the 45 minutes it takes to do the task and obtain 120 minutes for the LS, the same as
the ES since there is only one activity at this phase.
Next we move to the previous phase which consists of two activities: “Bag Grass”
and “Bundle the Trash”. These needs both to end latest at the 120 minute mark not to
delay the last phase. Thus their LF=120 minutes. Subtracting the time each activity
takes we end up with the LS for each one: 120 min-30 min = 90 min for “Bag Grass”
and 120 min-15 min=105 min for “Bundle Trash”. So bundling the trash could start
later than bagging the grass and not delay the whole process.
Next we move to the previous phase that has four activities in parallel: “Mow the
Backyard”, Trim Hedges”, Edge Sidewalk”, and “Trim Weeds”. All the LF values for
these are the same: the value of LS for “Bag Grass” the longest activity in the next
phase, that is 90 minutes. Subtracting the duration from the LF value we obtain the
LS value for each. The process is repeated till we reach phase one.
In the chain of activities, there is a path that has no slack or float. It is made of “PickUp Trash”, Mow Front Yard”, “Mow Backyard”, “Bag Grass”, and “Haul Trash”.
Any delay in any of these activities delays the whole project. It is the critical path:
the path of activities from beginning to end that has no slack. This is the one to
monitor carefully to make sure the project stays on time.
Now, to the issue of slack or float. Slack for an activity is the difference between the
ES and LS or the difference between EF and LF. Both have the same value.
What good is slack? Should people use it to start their activities late? That’s the last
useful thing to do! First of all, these durations for activities are estimates and may be
incorrect. Slack times should be used as insurance against bad estimates or
unforeseen problems.
Slack can be used to assign more resources such as people to the critical path
activities if that is useful (helps complete critical path activities earlier) and possible
(people do not overcrowd each other or newcomers do not actually slow down the
others by requiring instructions and coaching time). In our programs, slack will be
used to assign more people to critical path activities. We will ask each participant to
select a primary activity and a secondary activity. These will be assigned to make it
possible to shift personnel to critical path activities when slack occurs in their primary
activity.
How to shorten a task?
This is a very important question, in particular, for critical path activities. It is
possible to shorten a task by either
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Adding resources
Reducing the scope
Reducing performance
Being more efficient
Changing the process to a quicker one
We will not reduce performance but we may reduce the scope of work, add resources,
be more efficient, and look for a process that is quicker.
II. Convert the CPM Diagram to a Gantt Chart
Visually, a Bar Chart such as a Gantt Chart is a much better tool than an Arrow
Diagram. Arrow Diagrams are essential to do a proper analysis of the relationships
between activities in a project. That is why we started with an Arrow Diagram. We
now convert it to a Gantt Chart.
See Yard Work Bar Chart as an example of a Gantt Chart derived from an Arrow
Diagram.
2. Estimating Resources and Budgeting
This is the second round through scheduling. So far, we assumed unlimited resources.
It is time to be realistic and see what resources are needed to support the first draft of
the schedule and to modify it if necessary based on more realistic resources, or to
reduce the scope of work.
The most likely resource that is not available is enough people to do the work. Let’s
work through an example.
Consider the following project: it consists of four tasks: A, B, C, D. Tasks A & D are
estimated to require three weeks for completion when two people are assigned to
them. Tasks B & C require one person and have float. Task B requires 2 weeks of
work with one week of float. Task C requires 3 weeks of work with two weeks of
float.
With unlimited number of people the bar chart looks as shown below.
Now, there are limited resources: only three people are available to the project. Thus,
the Figure 1 picture below cannot represent the schedule as A, B & C together require
4 people working simultaneously the first two weeks. A solution is to shift C to start
at the beginning of week 3 instead of week 1. Then B becomes critical. It is better to
make B critical than C. Why?
Note that there are other solutions. Scope reduction is one of them. Changing the
deadline is another one. Finally, performance could be reduced.
In our case, we will not accept reduction of performance or shifting the deadline
further into the future. It is imperative that our projects be completed by the end of
the semester. Thus, we are left with reducing the scope.
Figure 1
Figure 2
A
A
B
B
C
C
D
D
Time in weeks
Time in weeks
In lab, you will use these concepts to finalize your schedule for the semester.
Budgeting for us is simple as we do not pay people to do work. The only costs are for
materials. You are given a budget and you will have to live within this budget.
Selection of materials will have to made as early as possible with an estimate for the
cost to make sure we stay within budget. If the budget shows cost overruns, we will
have to select less expensive materials.