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The Critical Path and Float
This is separated into two parts, with the first outlining the Critical Path, and the second doing the forward and backward pass on the critical path method. Now all of this it can seem a little daunting at first, that’s why we’ve just separated it into two videos, but ultimately this one will give us a broad overview and the next one we’ll delve into it in a little bit more detail.
What is the critical path?
The critical path is the longest sequence of activities that make up a path through a project – so it is the longest sequence of activities, and that determines how soon were able to actually complete our project, which gives us the shortest possible project duration.
It’s the path that doesn’t have any slack in it – so there’s no leeway, no room to move, and that’s why it’s the most critical path.
The critical path method is used to calculate the critical path in our project, and the amount of free float and total float, which is the flexibility that we have in our schedule. Float is the leeway that we might have – can we delay an activity by one day or two days? If there’s two days of float, then yes.
Let’s have a look at an example.
Let’s say we’re starting our project schedule, we’ve got these durations in the top middle of our schedule network diagram. In this case we’ve got the durations of 5 days, 5 days, 10 days for this one and 15 days for our last one. The float is described in the bottom middle box, so we’ve got zero days of float. In other words, there’s no leeway for this one there’s no leeway for this one and there’s no leeway for this. That means our critical path is A, C and D.
If you noticed we’ve got five days of float here, that means we could potentially delay this activity by 5 days, there’s a little bit of leeway and we’d still be able to get the project done at the same time. That’s the idea that we’re looking at.
So again broadly or at a high level, Free Float is the amount of time that a scheduled activity – so a single activity – can be delayed without delaying the early start of any of its future activities. Total float or project slack is measured by the amount of time that a scheduled activity can be delayed or extended without delaying the project finish date. So zero float, as we said, is shown on the critical path. There’s no leeway on that critical path, and that’s why it’s critical.
We will delve into this in more detail looking at the critical path method and the forward and backward pass to calculate all of these lovely values as part of your schedule model.
– David McLachlan