Critical vs longest path
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I have a paper posted on the web from a PMI College of Scheduling conference concerning this titled, "Longest Path Value." You can assess it at http://ronwinterconsulting.com/published.htm. Perhaps this will make it all clearer. Good luck!
CRITICAL PATH – is the longest continuous chain of activities (may be more than one path) which establishes the minimum overall project duration. A slippage or delay in completion of any activity by one time period will extend final completion correspondingly. The critical path by definition has no “float.”
LONGEST PATH (LP) – is the longest continuous path of activities through a project, which controls project early completion. It is possible for otherwise defined critical path activities to not be on the longest path and longest path activities to not show calculated critical float. The longest path analysis is unaffected by activity calendars. The longest path is determined by the string of activities, relationships, and lags that push the project to its latest, early finish date. The longest path is calculated by first performing a CPM ‘forward pass’ to determine driving relationships and the project’s latest, early finish date. The activity (or activities) with the latest, early finish dates are then identified and all predecessor driving relationships traced back to the project start date. These activities constitute the projects longest path. The longest path depends upon relationships driving the timing of activity starts, thus use of constraints and resource leveling can interrupt and invalidate longest path analysis. Use of interruptible activities can also result in false longest path indications. For complete accuracy, longest path analysis should take place absent of constraints, resource leveling, and/or interruptible activities.
critical path relies on tools-->options definition of total float, while longest path contains all the connected activities from the last activity (note: it might end at the middle).. in short, if it contains all the activities from start milestone, then it can be considered as critical path..