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Scheduling Fundamentals

Scheduling Fundamentals

There are several ways to represent a schedule. Let's look at some of them and learn what they show best. We'll look at the three most common forms of presenting a project schedule: table, Gantt chart, and network diagram.

Table

This is the simplest form for representing a project schedule. It is non-graphical and is just a list of the activities with a start and finish date. Other information can be provided, if desired. This is a great way to show a very long list of project activities, since the graphical methods tend to get messy with very large projects. A sample schedule table is illustrated in Figure 1.

A Sample Schedule Shown as a Table

Gantt Chart

The most frequently used schedule representation is the Gantt chart, sometimes called a bar chart. Invented by Henry L. Gantt during World War I, it was used to schedule the logistics of getting men and materials to the many ships on the U.S. coast for transport to Europe. In fact, you may already be familiar with it even if you've not been involved with any projects at all.

The Gantt chart, seen in Figure 2, is a simple illustration of activities listed down the left side, with an activity bar shown to the right under a scale representing time. True Gantt charts do not show dependencies, but many project-scheduling tools have options to draw dependency lines between activity bars. There is a variation of the Gantt chart called a milestone chart, which is just a Gantt chart with the activities omitted so it only shows the milestones.

Example of a Gantt Chart

Visually, the Gantt chart can represent the sequence of activities to be done on a project. The activity list can be sorted by start date (giving the familiar cascade look, down and to the right, as shown in Figure 2), by finish date (showing the order for presenting work products and deliverables), or by some other feature such as WBS code or level. For very large projects with many activities and lots of detail to show, Gantt charts can get cumbersome. For large projects, sometimes project managers print the schedules out on large format stock and "wallpaper" the office with them. This is great for seeing the "big picture" but is difficult to maintain.

Network Diagram

The network diagram, also called a logic chart, a PERT chart, or some combination of these, is a graphical representation showing an ordered list of symbols (usually boxes or circles) that indicate activity name and precedence, and often include other information, as well (such as duration estimate, start and finish dates, and responsibility). There are many different names for methods that use network diagrams, including:

●  GERT - graphical evaluation and review technique;
●  PERT - program evaluation and review technique;
●  CPM - critical path method;
●  PDM - precedence diagramming method;
●  ADM - arrow diagramming method.

The first three methods are regarded as analysis techniques. The last two are simply diagramming methods. Because PERT and CPM were invented at about the same time in the 1950s, they are sometimes lumped together and called PERT/CPM referring to a generic analysis process using a network diagram. GERT is also an analysis process, but it allows for conditional and probabilistic treatment of both precedence and duration estimates, making it very complex to use. PERT allows for probabilistic treatment of the activity duration estimates by using a three-point estimate instead of a single-point estimate, but assumes the network logic is fixed. CPM assumes that both the activity estimates and the precedence logic are fixed, making it the simplest to use. PERT and CPM use the same network diagramming approaches.

There are two basic ways to represent activities in a network. The basic network diagram is a collection of nodes and arrows. The basic information represented is:

●  Activity name or Node ID - often using the WBS code;
●  Earliest start - the earliest time period that the activity can start, based on prior activities' completions,
    or other project constraints (like fixed dates);
●  Earliest finish - the earliest time period that the activity can finish;
●  Duration - the number of time periods of work remaining for the activity;
●  Latest start - the latest time period that the activity can start without impacting the next milestone;
●  Latest finish - the latest time period that the activity can finish without impacting the next milestone.

One representation puts the activity information on nodes and is called an activity-on-node representation (AON). It is shown in Figure 3. The other puts the activity information on the arrows between the nodes and is called activity-on-arrow (AOA). It is shown in Figure 4. AOA representations may need to show dummy arrows ("activities" with no resources or time, generally shown as dashed lines) between nodes to indicate precedence when more than one activity follows. These dummy activities make reading AOA diagrams more complicated. PDM is an AON method, and ADM is an AOA method of diagramming.

AON Activity Information Representation


AOA Activity Information Representation

An AOA network is shown in Figure 5, and two AON networks are represented in Figure 6. AOA representations are seldom used today. Whichever network representation is used, the principles remain the same. For our purposes of network analysis, we will use the AON representation. In an AON network, there is a start node, generally placed on the left, and an end node, placed to the right. All the nodes in-between represent activities in the project WBS. There is a one-to-one correspondence of activities to network nodes. Although they could be represented as boxes, circles, or whatever shape is preferred for a node, we will represent nodes as shown in Figure 3.

Sample AON Network Diagrams


Sample AOA Network Diagram

You'll notice that all of these network diagrams show precedence very well. This is their major strength. It is easy to follow a path of activities from left to right, and to see how various activity sequences relate to each other. These diagrams are extensively used to construct project plans from scratch when no WBS template exists, that is, when you are inventing the project activities from a brainstorming session (as discussed in "Considering Dependencies"). The weakness of these diagrams is the same as for the Gantt and other graphical representations. For large projects with lots of activities, they get very messy and hard to follow. But they are a great way to get started with planning and to get a WBS under control.


Tags

project schedule, gantt chart, dependencies, logic chart, wbs
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