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Introduction to Scheduling
This paper will deal with the various scheduling techniques available
to the project manager, the technical staff, and the project planners.
The purpose is to determine which schedule technique should be
used, where it should be used, by whom it should be used, and
to which project phases it should be applied.
To make these decisions, we must first talk about the scheduling
process and then how to adapt the scheduling system to the scheduling
process. Scheduling, per se, is merely deciding in advance when
and where work will be performed - it is a TIME decision. However,
the scheduling process is usually connected with scheduling systems,
policies, techniques, and/or devices. In this context the scheduling
process centers around:
- Time to do the work,
- The department which will perform the work,
- The resources to be applied,
- Statusing work progress versus work scheduled, and
- Monitoring and reporting
In addition to the when and where decisions, the scheduling process
will involve procedural decisions and location decisions. This
will mean that planners, administrators, system engineers, project
engineers, project managers, etc. will make the schedule process
selection. Therefore, the process for scheduling must be a clear,
concise process, so that communication by all concerned people
can be accomplished at each management level.
Statusing and analyzing progress are the next aspects of the scheduling
process. This occurs after the schedule has been incorporated
into the total project plan and work has begun. Incorporating
progress data into the scheduling process is a mechanical step
which should provide the desired information for the final phase
of the scheduling process, monitoring and reporting.
It is somewhat difficult to quantitatively assess the utility
of one scheduling technique versus another without serving an
apprenticeship as a planner or master scheduler. To assist the
inexperienced user, the following criteria are presented as isolated
features which are desirable for evaluating various scheduling
techniques. The Project Office must assess how well the selected
technique satisfies the end goal by weights and indices with other
techniques.
Objective Criteria
A list of objective criteria judgments for review of a specific
scheduling technique to be used could include the following:
Accuracy
The system should provide accurate information, i.e., progress
reports should reflect genuine progress. Estimated remaining time
is better than percent time completed.
Reliability
Progress data should be consistent regardless of who collects
it or when it is collected. A system may be designed to provide
accurate information but, through weaknesses in data collection,
may not be reliable. Conversely, reliable yet invalid results
are possible.
Simplicity
A large number of people are likely to be involved in making entries
and drawing reports, graphs, and charts from a scheduling system.
Thus, the technique should be easy to explain, understand, and
operate.
Universality
Ideally, one scheduling system should be sufficient from beginning
to end of a project. All levels of management should be able to
use the information in the system, and all relevant control factors
should be encompassed by the one system.
Decision Analysis
Since management decision-making involves selecting one course
of action from alternatives, it is useful to assess scheduling
aspects of the alternatives. A system which enables management
to simulate the impact of alternative courses of action can make
decision making easier and result in better decisions.
Forecasting
One purpose of collecting data is to assess the ability to accomplish
future tasks on schedule. Some scheduling systems are better equipped
to provide this kind of advance information.
Updating
Project decisions in dynamic environments must be based on up-to-date
information. The scheduling system should be capable of rapidly
and easily incorporating information on project progress.
Flexibility
It is desirable that a scheduling technique easily adapt to changes
in the project.
Cost
The scheduling system should provide the required information
at the lowest cost. Cost is difficult to measure for several reasons.
First, total scheduling costs are needed to compare scheduling
techniques; but no one has reached an agreement on what costs
to include. For example, in a Gantt System, time standards are
as much a part of the cost as is chart preparation. Yet, frequently
this factor is not included in estimates of schedule cost, most
likely, because, time standards are used for other purposes. Secondly,
systems, which are most useful, generally cost more to operate.
Thus, the proper cost statistic is not total dollar cost but rather
cost per unit of utility or benefit. Cost per unit of utility
or benefit is next to impossible to measure. Finally, cost depends
largely on the size of the project and involves both fixed and
variable costs. Scheduling techniques with high fixed costs thus
tend to be more economical in large-scale then in small-scale
applications.
DETAIL DOCUMENTATION
of the
TECHNICAL SCOPE of WORK
All scheduling starts from a documentation of stated objectives.
The contract is normally the vehicle for identifying the stated
objectives. While this is a start, in order to use the contract
content in the day to day operation of the project, it must be
restated internally. This can be accomplished by a well defined
project plan, a work breakdown structure, and a work breakdown
structure dictionary.
MILESTONES
A milestone represents a product or an event. A milestone that
does not represent a meaningful product or event to the performer
is worthless.
Within the authority of the performer means that the individual
or group of people working toward the milestone goal must have
control over reaching that goal. We sometimes can share a milestone
- i.e. - Project Design Review (PDR) date could be shared between
several performers, but it must still remain within the control
of the performer.
A milestone which is not within the control of the performer generally
ends up causing problems. A simple example is when a Company Manager
tries to measure progress against a milestone calling for delivery
of an item by the Government (GFM/GFE) to the program. The Company
Manager cannot affect the delivery date only the Government representative
can affect the delivery date.
TRACEABILITY
PERT, Line-of-Balance, Gantt, milestone charts are all good techniques
which are effective when properly employed. Project scheduling
requirements basically seek formality, consistency and discipline
throughout the scheduling system regardless of the technique used.
All authorized work must be formally scheduled in a manner which
will permit the evaluation of actual progress against contract
milestones and which will identify interdependencies of individual
tasks. The schedule is necessary for developing a total project
plan and for the control of changes to the base project plan.
Specific information data sets are needed before a schedule can
be properly constructed. Work scope, ownership interrelationships,
time durations are all key sets of data needed prior to the beginning
of schedule development.
The scheduling system should contain summary or master schedules
which provide for all contractually specified milestones. The
summary schedule(s) should be clearly supported by lower level
schedules which link the summary to the detail tasks. All lower
level schedules must contain specific start and completion dates
which are based on physical accomplishment and are clearly integrable
with formal project or organization schedules.
The concept of traceability aids management in control of the
project by allowing for easy isolation to any area of project
if key milestones begin to move. It also helps when contract changes
or internal management directed changes effect the project baseline
Schedule Examples (Charts)
CHARTS
Flow charts or process charts do not normally have a time scale
and thus are not truly schedules. They do provide sequential relationships
of tasks to be performed thereby providing the planner/scheduler
with a task dependency orientation. Types of process charts include
procedure charts, the process-product chart and the process-man
chart. All process charts use geometric symbols to represent tasks
and straight lines to illustrate task sequence.
Leadtime charts or Set-back charts are simply a process chart
with a time scale reference. When the time scale is marked off
in minutes, hours, days, weeks, months, or years it provides visual
insight into activity durations. Great care is needed to assure
that all activities on a leadtime chart link together at their
respective times - making lead time charts more difficult to construct
than process charts.
Set-back charts do not show where the work will be performed and
therefore it is considered to belong more to family of process
charts then it does to the line-of-balance scheduling methodology
to be discussed later. In the manufacturing process when set-back
charts are used a standard set of geometric symbols are used to
indicate the resource function responsible for the activity.
Milestone Charts are normally used by first and second level management
to determine overall status on each major project. It is not a
good schedule methodology for day-to-day monitoring of work effort
at the detail level. Again, the need to define in writing what
constitutes completion of a milestone is required.
Bar Charts are intended for first and second level management
to determine overall status on each major project. It is not a
good schedule methodology for day-to-day monitoring of work effort
at the detailed level.
Gantt charts are the cornerstone of the Gantt technique of scheduling.
This was the first formal scheduling system to be used in conjunction
with scientific management. The Gantt chart normally looks at
first glance like a bar chart. It is however a little more detailed
in its representation of facts than we normally associate with
the common bar chart.
The Gantt chart normally is constructed with the time graduation
shown along the horizontal axis and personnel, organizations,
machines, materials shown along the vertical axis. The "open Bar"
shows the time units of work which are scheduled for each person,
organization, machine... the Gantt Chart is needed to portray
the initial schedule as well as to indicate current progress.
Modified Gantt / Milestone charts are an early attempt at indicating
a dependency relationship between key milestones. Once again,
however, one needs to document the dependencies and their respective
meanings between the key milestones.
Schedule Examples (Networks)
NETWORKS
All of the aforementioned concepts and methodologies pertaining
to scheduling lead to networks. The first recognized formal use
of a network schedule methodology was used by E.I. in Pont de
Nemours & Company in 1957 for new plant construction. The methodology
was called CPM or Critical Path Method. CPM, PERT, PEP, PRECEDENCE
are all network scheduling concepts. They each employ either the
ADM or the PDM concept.
ADM - Arrow Diagraming Method or AOA - Activity-on-Arrow is where the
task is represented by the line and the geometric figure (circle, square,
etc.) is the common connector between tasks. This is similar to the Leadtime
Chart Methodology when the activity is represented by the line.
Activities and events must first be understood. In ADM, the activity
is represented by the line. It means some sort of action is to
take place. It consumes time. The event, a geometric figure consumes
no time but represents the earliest start point or latest completion
point of any activity within the network. The event is the junction
points for all dependencies. An event, in contrast with an activity,
does notconsume time or resources. An event represents either the start
or the completion of an activity in the ADM methodology.
Real activities consume time. However, restraints or interdependencies
between events are shown as zero duration activities in ADM.
HAMMOCKlNG
Hammocks are used to show summaries of detailed logic as a simple
activity for higher level reporting purposes. Plain and simple,
the "hammocking" technique allows for detail network schedule
information to be collapsed into summary network work schedules
for senior management.
PDM - Precedence Diagraming Method, or activity-on-node is the second
concept for networks. The activity is described within the geometric
figure, and the lines are used for the connector device. This is similar to
the Flow Process Chart Methodology. PDM is the last major scheduling concept
developed for use on major complex projects.
The development of a precedence network following the fundamentals
of a flow diagram and allows the planner/scheduler to list all
the activities and show the interdependency links before time
estimates and resource requirements are defined.
The simplest PDM relationship is Finished to Start, and is similar
to the standard ADM activity relationship. PDM adds the additional
capability to build in a lag factor i.e., B cannot start until
some defined time period after the completion of A.
Another standard activity relationship is Start to Start. An example
of this type of activity arrangement would be as soon as the design
parameters and the engineering bill of material is released for
detail design the long lead procurement components can be given
to purchasing.
Finish to Finish relationship indicates that the completion of
activity B is dependent on the completion of activity A. If activity
B is the finish black box to be tested and activity A is the front
panel for the black box then this relationship indicates the front
panel must be available before the black box is complete and sent
to test.
MORE ABOUT NETWORK SCHEDULING
The following elements are needed for any network scheduling system.
We need a way to provide descriptive narratives for each activity,
then tie the activity to a calendar, resource, node module and
a relationship module. Then a procedure model for updating and
statusing the schedules. These elements are needed for either
methodology - ADM or PDM. All of this can be done by manual efforts,
but the use of a computer systems on complex projects will minimize
the manual clerical efforts.
Arrow Diagraming Method (ADM) is sometimes interchanged in the
scheduling language with Program Evaluation Review Technique (PERT)
and Critical Path Methodology (CPM).
PERT and CPM are refined techniques within schedule networks.
Sometimes we only need a network to illustrate activity interdependencies,
sometimes we want a schedule network with CPM and PERT.
CRITICAL PATH METHOD
Critical Path Method (CPM) looks at methodology for determining
the critical path. This methodology examines the network time
duration via activity duration calculations. CPM can be used for
either Arrow Diagramming Method (ADM) or Precedent Diagramming
Method (PDM). CPM is determined by utilizing the Forward or Backward
pass method.
A Forward pass through the network determines early start time
for each activity. Early Start Time defines the earliest an activity
can start, based on the defined schedule logic.
A Backward pass through the network determines the late finish
time for each activity. Late Finish Time defines the latest an
activity can be finish without delaying the project completion
date.
The longest duration of a set series of activities for the total
network will produce the critical path. It is possible to find
more than one critical path in a given network. If the network
is segmented into zones (areas) for analysis between two given
dates, the critical path may change. If work scope is added or
deleted from the project, the critical path may change. The critical
path will always show the shortest expected duration for completion
of a project, and if time durations are not met as planned, then
the project completion date will not be met.
DIRECTED DATE
A directed date is a specified date which is frozen in the schedule.
"Not earlier than" means that an activity can start no earlier than the specified date, even if the networksolution would allow it to start earlier. This type of directed
dates supersedes the network solution on the forward pass only.
Directed dates are normally specified by the customer, Senior
Corporate Management, the Project Manager, or the Functional Manager.
Directed dates normally result from the desire to integrate selected
items both internally and externally to the project.
This discussion does not mean to imply that direct dates have
no valid application. Any major milestones which must be fixed
should have imposed target dates. This will also help assure schedule
consistency for traceable milestones on successive reports. But
there are other valid reasons for using directed dates besides
using them to freezes major milestones. In come cases, a not later
than date should be imposed to assure that the activity is accomplished
during the necessary time frame.
PERT
Problems Eventually
Resolve Themselves!
Please see the PERT paper.
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