PMI says in exam - start with 0
By Barbee Davis, MA, PHR, PMP
When figuring the forward pass on a critical path, I start with zero. My colleagues say I should start with one, for the first day of the project. Each approach can give substantially different results, so which way is correct?
Answer: B. Start with zero to figure the critical path, as it is part of the set critical path formula.
Once the project activities have been defined, sequenced, and task durations estimated, a schedule is developed. However, it is important to the project manager to know how quickly this sequence of tasks could be completed under ideal circumstances: the critical path.
The Critical Path Method of Analysis (CPM) is a mathematical model you can use to calculate the longest path through the project, showing the shortest amount of time in which the project can be completed, with zero slack or float (wiggle room). The formula is 0 + Duration of A + Duration of B + Duration of C and so on, to complete the first of five calculations to figure critical path, a forward pass through the project. Assume the three activities are A (2 days duration), B (5 days duration), C (3 days duration).
Since projects never have simply a series of tasks to be done in sequence, you have to take into account activities done in parallel, or those which must be completed before other activities can start. To visually represent this, it is common to place the numbers on a network diagram. But remember, this is not showing which day of the project a task will be completed…this is just a visual image of the mathematical formula:
0 + A(2d) + B(5d) + C(3d) = 10days
If more than one task must be completed before you move forward, choose the largest number in the upper right corner of each activity, the early finish date, to move forward.
To complete the critical path calculations:
Now, you can show management how you could shorten the project schedule if you had:
But more importantly for the project manager, now you can place the project activities on a schedule with a firm start date and plot the days on which each task will actually occur. At this time you can include leads and lags, assign resources, and show resource constraints, employee vacations, overallocations and company holidays to make your project schedule a more realistic timeline than the artificial one shown by the CPM calculations.
If you are sitting for the Project Management Professional (PMP)® certification exam, remember Task A always starts at zero. It has no predecessor and, thus, can begin right away. You will get incorrect answers as you do the backwards path and float calculations otherwise.
Figuring the correct critical path is important for a project manager in order to find the critical tasks. Since a critical task has no slack or float, a delay or overrun for a task on the critical path will always delay the end of the project unless the project manager intervenes.
Barbee Davis, MA, PHR, PMP, is a reviewer for the global PMI Registered Education Provider Review Team. She owns Davis Consulting and is a published author, speaker, writer of training materials and an innovator in presentation skill workshops for corporate trainers. She holds a Black Belt in MS Project and teaches at the university level.
Her new book, 97 Things Every Project Manager Should Know, includes practical tips from experienced project managers around the world. Ms. Davis is available for speaking engagements and encourages your questions or comments.
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