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  #1  
Old 06-16-2011, 06:14 AM
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Difference in Lag/Lead and Float on a Network Diagram

Is the following a correct explaination of Lead/Lag time in reference to a Network Diagram (Critial Path). Thank you all in advance.

Lead/lag times are not really shown seperately on the network diagram.

Rather they would just be part of the Duration Figure of an activity. Meaning, when looking at he network diagram you would not know which activity had lead or lag,

Float would then be determined by Start/Finish and Duration of an activity. Lead/Lag would simply be incorporated in the Duration figure of that activity?

One more for you! I am correct about the following too?

•Free slack This is the total time a single activity can be delayed without delaying the early start of any successor activities.
In summary - Free Slack only impact a successor activuty.

•Total slack This is the total time an activity can be delayed without delaying project completion.
In summary - Total Slack must impact a successor activuty (or more) which results in project delay?
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Old 07-23-2011, 06:00 AM
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Hi There!

First up, your definition of free and total slack are correct. Be aware that *sometimes* free and total slack can be the same (for example, say, 3 days before the successor task is impacted, and the successor is on the critical path and so the end date slips (total slack also = 3 days)

Okay, onto your first question, which is a little ambiguous, but I think i know what you mean....

Let me clarify what these are first: Lag time is a time delay between the predecessor task finishing and the successor task starting. For example waiting for cement to set before you can mount something on it. It's just a time delay.

Whereas Lead time is the amount of time that a predecessor and successor task 'overlap' in time. For example you can start cable installation 2 days before the painting has been completed.

So here's my answer. Lead and Lag must be explicitly stated and shown in some way.

Now on a Network diagram, the only thing you would see are the shapes/boxes shown for each task, and the linkages (dependencies) between each task. Depending upon the software you are using, such lag/lead information could be added to the diagram.

In terms of the calculation by the software tool, lead and lag values will have an affect on the calculation (including slack) of start/finish dates.

By the way, a Gantt diagram will explicitly show leads/lags by virtue of the tasks being shown against a backdrop of a timescale.

Lead/Lags are never part of a task duration - they only show that task's duration in time reference to its other connected tasks.

If you don't like my answer (!!) then an alternative is to split each task into two separate tasks for leads, and treat them as separate tasks, one to feed into the successor, and the other, in series, to represent that portion that is still ongoing while the successor starts. And to create two tasks for a lag, where the second task would be called, for example, "wait for cement to dry", and would obviously have no resources assigned!

I deal with such aspects within [URL="http://www.project-management-primer.com"]THIS[/URL] training product
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Last edited by Dave Litten; 07-23-2011 at 06:07 AM.
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Old 07-27-2011, 11:57 AM
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Use Gantt in place of network diagram

I am not sure why you would like to use network diagram for this. Gantt is much better suited for showing lead/ lag and other timing constrains.

David R. [URL="http://www.binfire.com"]online project collaboration[/URL]
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Old 01-25-2013, 12:32 AM
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how do u deal with lag /lead when making a network diagram manually ??
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