Precedence Diagram | How to create a precedence diagram

a simple precedence diagram

This is a guide to project planning covering 4 stages. If you haven't read steps 1, 3 or 4, you can access them here. Stage 1 - Work Breakdown Structures, Stage 3 - estimating effort and scheduling, Stage 4 - resource allocation and levelling.

Stage 2 of project planning focuses on the order in which the tasks need to happen. To understand that we take need to create a Precedence Diagram

The first step in planning is to breakdown the tasks that are needed to deliver the sum total of the project in a Work Breakdown Structure. Knowing the tasks that need to be done is essential, but you also need to know in what order they need to be done. To do this we take need to create a Precedence Diagram

How to create a Precedence Diagram

  1. Tape some flipchart paper to the wall or use whiteboard.

  2. Put the top deliverable from your work breakdown structure on to the right hand side of the board or paper.

  3. Take each of the lowest level post-its from your breakdown structure and arrange them in the order in which they need to happen.

  4. Work from the left until you the dependencies between the tasks and you have a sequence of tasks running from left to right.

Focus on the sequence of the tasks. Be careful not to think about resources or other constraints. Just think about what order the tasks could go in assuming no restrictions. As you go through this exercise you may identify additional tasks - add them to the precedence diagram and to the work breakdown structure. Finally draw arrows between the tasks that link up, and make sure that there aren’t any tasks sitting isolated. You should have a flipchart or whiteboard that looks like this.

precedence diagram using post-its

This is fantastic because this shows you the dependencies between tasks and it enables you to put together a critical path for your project. Don’t worry about calculating the critical path MS Project can do that for you. The point is you can’t calculate an accurate critical path unless you have been through this process. This article explains what I mean by the critical path.

I strongly suggest that you go through this stage in a collaborative fashion with your team because you are not necessarily going to know what all of the dependencies are.

By now the meeting room will probably be covered in post-its and you need to transfer it all into your scheduling software. You can either stay in the room or take a series photographs. I tend to stay in the room so that I can get everything in to MS Project and be sure that I haven’t missed any dependencies.

Type all of the tasks into your project scheduling software. As far as possible, try to follow the logical sequence on your precedence diagram. The great thing about the precedence diagram is that it provides you with the information you need to correctly link the tasks in the planning software. Make sure each task is correctly associated with its predecessors. Most scheduling software will enable you to link tasks so that you can accurately reflect the sequence in your diagram.

Task Dependencies

Microsoft Project offers four ways to illustrate task dependencies (link types). The diagram below illustrates each task dependency type.

Task dependency types in Project Planning

Don't worry about resource availability at this stage. If you are using Microsoft Project you can check that you have the correct sequence in the Gantt chart view. I suggest that you always check the dependencies for your project carefully as mistakes in the logic can prove very costly. Every Project Manager will have experienced that moment of panic when they realise that a task link is missing, and on adding the predecessor they find that the project end date moves to month later than planned.

precedence diagram in gantt view

Now we have broken down the work and identified the Task Dependencies it is time to estimate the effort for each task.

Stage 3 - Estimate effort