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When Do You Need One of These Tools?
For simple projects with few activities and dependencies, the critical path can be easily be determined by the project manager eyeballing the list of activities displayed in a precedence or project network diagram. However, as the total number of tasks and dependencies increases, the project manager will need to turn to a critical path method calculator to quickly and accurately generate a report that will indicate the shortest time in which a project can be completed, the critical path of the project, and the slack time associated with any of the activities. From the critical path method (CPM) analysis of this report, the project manager can then make better decisions regarding resources needed to execute a project and the most efficient way of shortening the time if the project's schedule needs to be compressed or crashed.
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Sporkforge offers a free online critical path method calculator that has the look and feel of a standard calculator. To operate this calculator, the project manager manually enters the name of each task, the task's expected duration, and the name of any predecessor tasks. However, information must be entered precisely as follows or the CPM calculator will fail to return any results.
- Quotes must be used around a task name that contains any spaces or tabs. (For example "Task A" versus A, where the quote would be optional).
- Duration time units must be generic (1, 2, 3, etc.) and uniform for all task entries. The start times reported in the results assume a project start time of 0.
- The task list must be entered in the text box provided, or uploaded as a text file in the same format. There is a 5 megabyte file size limit.
- This tool has a limit of 1,000 project tasks.
Microsoft Excel - With a little ingenuity, you can also create a critical path method calculator in an Excel spreadsheet. This article on use of Excel as a CPM calculator walks you through the steps and it only takes a few minutes to set up the template. After entering the tasks and designating the six different paths in our example project, the Excel spreadsheet produces the same critical path information as the Sporkforge calculator and has this appearance.
Microsoft Project creates a similar report with the added bonus of a having a Gantt chart to visually display the critical path and slack time shown in red and by line extensions, respectively. Microsoft Project also makes it easy to insert critical path milestones to highlight these noteworthy achievements. To create the critical path diagram, select the Gantt chart view, enter the names of the tasks, the duration of each task, the predecessors (located under advanced settings), and then check the boxes for the critical path and slack time. The results of our example are displayed in this format.
You can also generate a summary report by selecting the critical path from the reports catalog.
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Limitations of a Critical Path Method Calculator
An effective critical path method analysis can make the difference between success or failure on complex projects. However, a critical method calculator is only as good as the information that you have inputted. If a predecessor task is not identified or the duration of an activity is underestimated, the project's duration and the critical path will not be correct. To improve the time estimates, project managers often use a PERT formula which is a variation of the CPM analysis that factors in the most optimistic and pessimistic finish times for each activity but also gives more weight to the most likely finish time. Also, a critical path method is not designed to account for tasks that share the same resources that could add further constraints. For these calculations, you will need to switch to a more advanced scheduling calculator or a program application that addresses these contingencies.
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Image Credit: Critical Path Algorithm/Petr Kopač at http://commons.wikimedia.org/wiki/File:Critical_path_algorithm.svg
Screenshots taken by Ginny Edwards