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The critical path is the set of all the tasks which if delayed by the scheduling quantum will delay the completion of the project. When you work your way backward from the end to the beginning, if you come to an "air gap" in the timeline, whatever is between that point and the beginning of the timeline is not on the critical path unless something prior gets delayed by enough to close the gap. If I have to do something by Friday, but can't start the final step until Wednesday (and it will take all of my time from that point until the deadline to complete it), the steps I can do on Monday are not on the critical path unless I don't get them done before Wednesday. I can blow them off until Tuesday without impacting the completion of the sequence on Friday. They are not on the critical path of the schedule. The task on Wednesday will delay the completion on Friday if it starts late, so it is on the critical path.

I do agree that it would be nice to be able to tell OmniPlan that such gaps below a certain length should be treated as non-existent so that you can more readily determine what is likely to be the effective critical path. If Joe Programmer has to finish his widget prior to integration starting Monday morning at 8AM, the fact that he's scheduled to complete his work the preceding Friday at 4:55PM doesn't leave much room in the schedule for unpleasant surprises before his task really is on the critical path. On the other hand, if he's scheduled to complete his work two weeks earlier, he could do quite a bit of extra posting on the Omni forums without affecting the date the product ships. A suitable value for the fuzz factor is clearly going to depend on the project plan; what is a suitable value for the multi-year OmniPlan 2.0 effort is going to be a bigger value than would be appropriate for my two-day plan for preparing a big holiday feast.

You can (tediously) work around this behavior of OmniPlan's by putting in dummy elements before and after such a gap and linking them with dependencies. This allows you to run the critical path around the item with the fixed start date. I've attached a sample file showing an example of this, and two screen shots (one with the dummy elements, one without).
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Attached Files
File Type: zip OmniPlan critical path example.zip (25.0 KB, 269 views)