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The 12 Crimes of Critical Path Diagrams
By Chris Croft

Network diagrams are like a sort of flow diagram of the tasks in a project so you can see the running order of what depends on what. You can use a computer but post-its are best. Normally they are drawn across the page, left to right. The objective is to find the longest path, or ‘critical’ path, which tells you how long the project will take.

Common mistakes made by people doing network diagrams with post-its (or on a whiteboard):

  1. Vertical lines. This is a sin because it’s not clear whether the line is going up or down. Lines should always go diagonally across – makes the diagram much easier to understand. Sometimes vertical lines are used to show that the tasks happen together – but in this case the two tasks should both feed from the one on their left and into the one on the right.

  2. Arrows going backwards (or forwards then backwards then forwards) – you must move the boxes so the arrows always flow to the right. It makes it much easier to see the flow of the project. Similarly arrows which cross over make the project much harder to “see”, though very occasionally these cannot be avoided.

  3. Dangle. Every task should have at least one arrow coming into it and one coming out of it. If it has no arrow coming out of it then why are you doing it? At the very least, the arrow should go to “end”.

  4. Arrows coming out of the start of a box, or into the end of a box. This is confusing – they should be drawn coming out of the end of one box and into the beginning of the next, from left to right. Yes I know you might want to show lag, e.g. we want to start the next task half way through this one, but see next sin:

  5. Not granular enough. If you want to start the next task half way through this one, then you need to break the first task into two. Then, after the first half, you can show arrows going to the second half, and also to the next task.

  6. Redundant arrows. This is getting tricky to describe in words alone, but I hope you’re still with me! If you can’t pour the tea until you’ve boiled the kettle, and you can’t boil the kettle until you’ve filled it, you don’t need another arrow from fill kettle to pour tea. Redundant arrows are often easy to spot since they form a triangle.

  7. Loops. You can never have arrows that go backwards (ie right to left) and if you do then you run the risk of having an infinite loop.

  8. One task much too big. If most of the tasks are a couple of weeks and there is one that is 8 months, then you probably need to granulate the big one: break it into smaller bits.

  9. Tasks of one day. I don’t believe anything happens that quickly. Especially not several in a row.

  10. Mixed units – if you have some durations in days and some in weeks, or months, the adding up will probably go wrong. The whole point of the network diagram is to make the project instantly visible.

  11. Too series or too parallel. After a while you just know when a critical path chart doesn’t look right. It should be a mix of parallel and series tasks. Too liner = too slow, too parallel = too risky and needs too many resources.

  12. “Ongoing”. All tasks must have a start and a finish. If you can’t do this for a task and you find yourself wanting to say the forbidden O word then it needs to be broken down further.

Chris Croft runs and is one of the UK’s leading trainers and provides a wide variety of courses designed to involve, inspire and motivate people of all levels. You can read more from Chris on his blog.

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