Fundamentals of Scheduling

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OLE OF THE

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ANAGER AND

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he project manager plays an important role in project scheduling. The development of a realistic schedule is crucial to the project’s overall success. The project manager needs to establish checkpoints and milestones to insure the project is kept on track. Insuring that the overall project is completed on time is critical to a project manager’s success The importance of completing a project “on-time” has great financial consequences. Many clients include in their contracts a “bonus” or penalty depending on the projects overall completion date.

The purpose of this chapter is to review the fundamentals of scheduling which provide the basis for today’s project management software programs.

Computer tools for project management are discussed in Chapter 4.

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(CPM), P

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(PERT)

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^HART

CPM, PERT and Gantt Charts are various methods used to manage project schedules. This chapter will focus mainly on the Critical Path Method of Scheduling.

The Critical Path Charts are similar to PERT Charts and are sometimes referred to as PERT/CPM.

On the other hand a Gantt chart is a matrix which lists on the vertical axis all the tasks to be performed. The horizontal axis is headed by columns indicating task duration.

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^ISTORICAL

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^ACKGROUND

CPM scheduling was developed in the late 1950’s. It was introduced to the industry as a tool to improve planning and scheduling of construction programs. Concurrent with industrial development of CPM, the U.S. Navy introduced a similar method of scheduling called PERT. PERT is an acronym for Program Evaluation and Review Technique. The Navy developed this method to evaluate and monitor progress of the Polaris Missile Program. The major difference between CPM and PERT is that PERT is a more probabilistic approach that lends itself to activities for which there is little or no historical experience, whereas CPM uses historical information for establishing durations. Subsequent development led to a considerable amalgamation of the two methods.

It was not until 1967 that James Kelly developed the tech- niques of CPM as used today. He used digital computer tech- niques developed by Rand Corporation and applied them to a complex construction project for DuPont Corporation. This re- sulted in completion of a project well ahead of schedule.

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BJECTIVES OF

CPM

Figure 3-1 lists the objectives of CPM scheduling. As seen from the figure, CPM can be used as a logic tool for decision- making. It provides a means for planning, scheduling, controlling and presenting alternate courses of action. It also provides a visual means of communication to Project Management and an or-

ganized approach to implement a schedule program. CPM scheduling can be carried out manually or with a computer program.

A major problem with the CPM computer programs can be the number of activities. Very large networks became the norm during the 1960’s. Size, not quality, became a dominant factor and computer scheduling methods became more important than the scheduling program itself. Theory replaced practicality and, as a result, quality of scheduling deteriorated.

It was not until the mid-1970’s that a proper balance of computer method and size of networks was achieved. Experience has shown than 10,000/20,000 activity networks are costly, unmanage- able and inefficient. Careful prior evaluation of criticality and networks with a maximum of 5,000 activities have proven effective.

Figure 3-1. Objectives of CPM

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• Plan • Communicate

• Schedule • Organize

• Control • Implement

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^{ERMS AND}

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EFINITIONS

Figure 3-2 lists terms and definitions of typical CPM schedules. Brief definitions of each are covered with further explana- tions to follow.

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^IAGRAMS

Figure 3-3 shows three methods of drawing CPM diagrams.

Each has its pros and cons.

Arrow Diagramming, at present, seems to be the most popu- lar method. This probably stems from the fact that it was the first

Figure 3-2. Terms and Definitions

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Activities (arrows) An item of work, with or without its duration.

Nodes (events) Start and finish points of an activity.

Arrow Diagram A Network showing a logical sequence of activities and events which are graphically shown as arrows and nodes.

Restraints Limiting activities that prevent other activities from starting. They are non-time consuming and are referred to as “dummy” or dependent activities.

Critical Path The longest duration chain in a Network.

Early Start (ES) As implied this is the earliest time that work can begin on a given activity.

Late Start (LS) The latest time that a given activity can start without affecting the overall project duration.

Early Finish (EF) The finish achieved by starting a given activity at its Early Start and achieving the estimated duration of that activity.

Late Finish (LF) The latest time that an activity can finish without affecting the overall Project Duration.

Float Spare time available to activities not on the Criti- cal Path.

Total Float The amount of spare time available to an activity if all preceding activities are started as early as possible and all following are started as late as possible.

Free Float The spare time available to an activity when all activities in the chain are started as early as possible.

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method to be developed and computerized. It is also easier to associate with time and flow of job activities.

A major difficulty to arrow diagramming is the “dummy”

activity. Learning the significance and proper usage of “dummies”

requires time and experience. The arrow diagram is also cumber- some to modify.

The second method is Precedence Diagramming. As shown, the activities are on nodes. Length and direction of the arrows have no significance as they indicate only the dependency of one activity on another. This method is commonly referred to as “Ac- tivity-on Node.”

This method has received wider acceptance over recent years. Its primary advantage is that it eliminates “dummy” activities. It is also easy to modify. Since there are no events in the “Activity-on Node” diagrams, it is difficult to use milestones in the network; therefore, visual aspects of precedence networks are poor. As there is no dateline, it is also very difficult to view overall status.

Both methods are acceptable, however, arrow diagrams continue to have the slight edge because of early acceptance and familiarity.

The third method, showing a time-scaled network, is just a more “visual” tool of the arrow diagram. It is not designed as a tool for detailed control, but a technique to present overall schedules to management. It gives a quick and simple picture of the schedule as it relates to time, activity interfaces and criticality.

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^ETWORK

Figure 3-4 illustrates a simple network of an arrow diagram.

There are three activities: A, B and C. They can be defined as follows: A is the beginning activity; B follows A but cannot begin until A is complete; and C is the final activity following the completion of B. As shown, there is a logical sequence of work starting from left to right.

Figure 3-3. CPM Drawing Methods

Figure 3-4. Simple Network

Problem 3-1: Network Development

In order to develop a network, the following example illustrates the steps involved. Given the data as indicated in Figure 3- 5, draw an appropriate network.

Activities must follow in a logical sequence.

Analysis

First, read through the given data and note that this in a nine- activity network. Activity A is the first activity and Activity I is the last.

Figure 3-5. Network Development Problem

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Given:

1) Activity B follows activity A

2) Activity A is the beginning activity 3) Activity C follows activity A

4) Activity C precedes activities E & F 5) Activity D follows activity B

6) Activity G follows activities D & E 7) Activity F precedes activity H

8) Activities G and H precede activity I 9) Activity I is the last activity

Draw the appropriate network.

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Figure 3-6 shows the completed network diagram. Networks become more complex as activities are added and durations are established for each activity. Activity durations can be in days, weeks, or months; therefore, it is essential to determine from the outset the time scale. Logical sequence and durations for each activity can be determined by past experience or by work content in relation to available resources. This determination should evolve from consultation between the scheduler and appropriate construction and engineering personnel. It is important that an operating group concur with the schedule development, accept it as their schedule, and make a commitment to operate as per the plan and schedule. Work sequence can then be checked and durations assigned to each activity.

Today the project manager can evaluate complex schedules using a wealth of software available.

Figure 3-6. Network Development Solution

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