Develop Project Schedules by Analyzing Network Diagrams

Project managers develop project schedules by analyzing network diagrams — a skill that may seem overwhelming at first. Knowing how to analyze network diagrams requires a little patience and understanding the purpose of the two types of analysis required: the forward pass and the backward pass.

  • Forward pass: Determines critical paths, noncritical paths, and earliest start and finish dates

  • Backward pass: Determines latest start and finish dates and slack times

Analyze network diagrams with the forward pass

Your first step in analyzing your project’s network diagram is to start at the beginning and see how quickly you can complete the activities along each path. This start-to-finish analysis is called the forward pass.

To help you understand what a forward pass is, you can perform one through the diagram here. First, consider Activities 1 and 2 on the upper path:

Consider working on either Activity 1 or Activity 3 as soon as the project starts.
Consider working on either Activity 1 or Activity 3 as soon as the project starts.
  • The earliest you can start Activity 1 is the moment the project starts (the beginning of week 1).

  • The earliest you can finish Activity 1 is the end of week 5 (add Activity 1’s estimated duration of five weeks to its earliest start time, which is the start of the project).

  • The earliest you can start Activity 2 is the beginning of week 6 because the arrow from Activity 1 is the only one leading to Activity 2.

  • The earliest you can finish Activity 2 is the end of week 6 (add Activity 2’s estimated duration of one week to its earliest start time at the beginning of week 6).

  • The earliest you can start Activity 3 is the moment the project starts (the beginning of week 1).

  • The earliest you can finish Activity 3 is the end of week 1.

  • The earliest you can start Activity 4 is the beginning of week 2.

  • The earliest you can finish Activity 4 is the end of week 4.

  • The earliest you can start Activity 5 is the beginning of week 6.

  • The earliest you can finish Activity 5 is the end of week 7.

  • The earliest you can finish Activity 2 is the end of week 6. Therefore, the earliest you can finish the entire project (and reach the milestone called End) is the end of week 7.

So far, you have the following information about your project:

  • The length of the critical path (the shortest time in which you can complete the project) is seven weeks. Only one critical path takes seven weeks; it includes the milestone Start, Activity 1, Activity 5, and the milestone End.

  • Activity 2, Activity 3, and Activity 4 aren’t on critical paths.

Analyze network diagrams with the backward pass

In case resource conflicts or unexpected delays prevent you from beginning all the project activities at their earliest possible start times, you want to know how much you can delay the activities along each path and still finish the project at the earliest possible date. This finish-to-start analysis is called the backward pass.

You found out from the forward pass that the earliest date you can reach the milestone End is the end of week 7. However, you can’t reach the milestone End until you’ve completed Activities 2 and 5. Therefore, to finish your project by the end of week 7, the latest you can finish Activities 2 and 5 is the end of week 7. Therefore,

  • You must start Activity 5 by the beginning of week 6 to finish it by the end of week 7 (because Activity 5’s estimated duration is two weeks).

  • According to Rule 2, you can’t start Activity 5 until you finish Activities 1 and 4. So, you must finish Activities 1 and 4 by the end of week 5.

  • You must start Activity 4 by the beginning of week 3.

  • You must finish Activity 3 before you can work on Activity 4. Therefore, you must finish Activity 3 by the end of week 2.

  • You must start Activity 3 by the beginning of week 2.

  • You must start Activity 2 by the beginning of week 7.

  • You can’t work on Activity 2 until you finish Activity 1. Therefore, you must finish Activity 1 by the end of week 6.

You must finish Activity 1 by the end of week 5 to start Activity 5 at the beginning of week 6. But, to start work on Activity 2 at the beginning of week 7, you must finish Activity 1 by the end of week 6. So, finishing Activity 1 by the end of week 5 satisfies both requirements.

To organize the dates you calculate in the forward and backward passes, consider writing the earliest and latest start dates and the earliest and latest finish dates at the top of each milestone or activity box in the project’s network diagram.

A network diagram with earliest and latest start and finish dates.
A network diagram with earliest and latest start and finish dates.
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