To draw your project’s network diagram, you first have to decide the order of your project’s activities and that means setting precedence. Although some tasks have to be performed in a specific order, others don’t.
A predecessor to an activity is an activity or milestone that determines when work on Activity A can begin. The following four relationships can exist between a predecessor and the activity or milestone coming immediately after it (termed its successor):
Finish-to-start: The predecessor must finish before the successor can start.
Finish-to-finish: The predecessor must finish before the successor can finish.
Start-to-start: The predecessor must start before the successor can start.
Start-to-finish: The predecessor must start before the successor can finish.
An activity is an immediate predecessor to Activity A if you don’t have any other activities between it and Activity A. When you determine the immediate predecessors for every activity, you have all the information you need to draw your project’s network diagram. The following considerations affect the order in which you must perform your project’s activities:
Mandatory dependencies: These relationships must be observed if project work is to be a success. They include
Legal requirements: Federal, state, and local laws or regulations require that certain activities be done before others.
Procedural requirements: Company policies and procedures require that certain activities be done before others.
Hard logic: Certain processes must logically occur before others. For example, when building a house, you must pour the concrete for the foundation before you erect the frame.
Discretionary dependencies: You may choose to establish these relationships between activities; they aren’t required. They include
Logical dependencies: Performing certain activities before others sometimes seems to make the most sense.
Managerial choices: Sometimes you make arbitrary decisions to work on certain activities before others.
External dependencies: Starting a project activity may require that an activity outside the project be completed. For example, imagine that your project includes an activity to test a device you’re developing. You want to start testing right away, but you can’t start this activity until your organization’s test laboratory receives and installs a new piece of test equipment they plan to order.
You can decide on the immediate predecessors for your project’s activities in one of two ways:
Front-to-back: Start with the activities you can perform as soon as your project begins and work your way through to the end. To use this method, follow these steps:
1. Select the first activity or activities to perform as soon as your project starts.
2. Decide which activity or activities you can perform when you finish the first ones (from Step 1).
3. Continue in this way until you’ve considered all activities in the project.
Back-to-front: Choose the activity or activities that will be done last on the project and continue backward toward the beginning. To use this method, follow these steps:
1. Identify the last project activity or activities you will conduct.
2. Decide which activity or activities you must complete right before you can start to work on the last activities (from Step 1).
3. Continue in this manner until you’ve considered all activities in your project.
Regardless of which method you use to find your project’s immediate predecessors, record the immediate predecessors in a simple table.
When you create your network diagram for simple projects, consider writing the names of your activities and milestones on sticky-back notes and attaching them to chart paper or a wall.
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Source:http://www.dummies.com/how-to/content/how-to-determine-precedence-in-your-projects-netwo.html
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