Introduction
to Scheduling
Objective Criteria
Important Elements for
Every Schedule
Schedule Examples (Charts)
Schedule Examples (Networks)
More about Network Schedules
Back
to Project Management
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:
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.
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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 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.
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.
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
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.
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.
Hammocing:
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.
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 (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.
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.
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