Can anybody help me with this issue?
When and how I have to apply Normal and Beta distribution in the project showed in the attached document, I try to determine the total estimated duration and I came across this question.
The doubt is, MSproject calculate 229 days for that project but Gido & Clements book about this topic (Chapter 6) said that the total sum of the estimated durations must be used to calculate the total estimated time for the project (Have I misunderstood?), It has no sense for me because if we had e.g. 99 start-start tasks and 1 Finish-Start task (only one predecessor) and all of the have 5 days of estimated duration, then the critical path indicate that we have 10 days of a total duration of the entire project, but if we have to sum all estimated task durations It would show 500 day estimated duration project and it was a really innaccurate estimation
Thanks
PD: Document's language=Spanish Saving Changes...
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Wai Mun KooPMO Director| Intergraph PP&MSingapore, Singapore
Cristian, let me try to answer your question. I might not have fully undertstood your question correctly but I will try.
First there are two key topics here - 1) Task dependency and 2) duration vs. effort. Both topics are closely related.
Let's take your example as a reference. There are 99 tasks with start-start dependency which means that "a task cannot start beofore another task starts". Since you did not mention how the dependency among the 99 tasks are linked, I will assume that 98 of the tasks can only start when the first task has started. I further assume that they all start at the same time and take around 5 days to complete since you have not mentioned the start and end date for each of them. This will make the first 99 start-start tasks to complete within 5 days. Next, you have 1 task that has a finish-start dependency so it can only start after the previous start has finished. As this task takes 5 days to complete, this will result in the project duration to be 10 days (5+5).
There is one key concept that we need to take note - effort vs. duration. Effort is how much time per resource is required to complete a task and therefore, it is commonly stated in Man-day. Duration is the time difference between the start and end dates.
I can have a task that needs 100 Man-days effort to complete. If I put in 5 resources to work on it (assuming we can distribute the workload evenly and no overlap), I can have the task completed within 20 days duration (100 / 5 = 20).
Similarly, imagine I have 5 tasks each of 3 Man-days effort, then the total effort required to complete all the tasks will be 15 Man-days (5 x 3 = 15). If all the tasks have finish-start dependency (no other lead/lack in between), I will need 15 days duration to complete all the tasks (effort=duration in this case). If all the tasks do not have any dependency and I am able to start them anytime I wish, I may now plan all of them to start at the same time. However, I can only do so if I have 5 resources available so that I can assign each of them to work on one of the tasks. In this case, the total effort will still remain as 15 Man-days but the duration or the time I need to complete all the tasks can be shorten to 3 days (since each task needs 3 days to complete and I can start all of them together). If I have only 1 resource available, then I can't plan all the tasks to start together since I do not have enough resources to work on all the tasks at the same time. In this case, the only 1 resource I have will have to work on each task at a time and complete the current task before moving to the next task. Therefore, this will end up taking 15 days duration to complete all the tasks.
In short, there are a lot of factors that will affect the duration of the task, e.g. resource availability, task dependency and how you plan your task schedule. Duration is strictly a time unit while effort is related to work-done.
Hope the above explanation is clear enough. Saving Changes...
There are 2 types of total duration considered for a project
1. The Critical Path duration: marks the calendar days project duration
2. The total effort duration: define the project duration measured in days of total effort
These 2 durations are very different, the question is
Which duration I consider to develope the PERT technique?
an another: if i need to calculate the critical path duration with a probability of success of 84%, How do I add the total standard deviation to the duration if I need to sum the variances of each task and then total variance serve me to obtain the standard deviation?
With this I add 1 standard deviation to calculate the total duration whit the probability of 84%, but... which duration I choose?
thanks Saving Changes...
Wai Mun KooPMO Director| Intergraph PP&MSingapore, Singapore
You should use the critical path duration since it is the minimum possible time you need to complete your project. Saving Changes...
