In this PM training video, Devin Deen, project management expert and trainer, discusses how to calculate task and activity length and duration to create better project schedules.
Here’s a shot of the whiteboard for your reference!
In Review: How to Estimate Tasks and Dependencies
As Devin said, estimating time on a project is a bit of a guessing game. And he noted three different things that were important to remember when you approach this project management art.
- People aren’t great at estimating activity task duration
- People tend to fill the available time with the effort required to do the task
- Always plan for slack on your project schedule
That said, Devin did walk through a number of ways to do the best you can to estimate how long a task may take on your project. Even if you can’t be certain as to its duration, it’s your job to calculate all the variables to the best of your ability and have a window in which you’re pretty sure that work will be completed.
Pro-Tip: When working on an activity estimation, remember that your resources may not be great at estimating the time that their tasks will take. People, as noted above, will use the time allotted rather than the time a piece of work in fact takes. That’s why it’s crucial that you factor this in and give your deadlines some breathing room or else be stuck with an unfinished task.
Take it further and Train Your Team to Estimate! which may help you get more accurate timing on task completions.
Thanks for watching!
Hi and welcome. I’m Devin Deen, Content Director here at ProjectManager.com. Today’s whiteboard session is a bit of a guessing game. Specifically, it’s about estimating activity time.
Now, as a project manager, I’ve recognized three different things which are important to remember when you’re estimating activity time. First off, people aren’t really that great at estimating activity task duration. Second off, people tend to fill the available time with the effort required to do a task. Much like a goldfish swells to its bowl, people tend to fill in that available time they have to complete a task. That’s called Parkinson’s Law. Lastly, to have a reasonable chance at achieving a project deadline, project managers should definitely plan in slack in their project schedule.
All right, let’s get to the task at hand, which is estimating the effort for a particular task.
What I like to do is get my project team together in a workshop and list out all the different activities that we have to achieve and get them to tell me as a group what the optimistic and then pessimistic estimates are for achieving that particular task. When we’re going through enlisting the optimistic items, I take note of all the things, and I ask them all the dependencies that are required to actually achieve that optimistic estimate.
When we’re going through and doing the pessimistic estimate for that particular activity, I then get them to tell me, what are the things that can go wrong? And I list those items on the risk register and make sure that I have mitigation strategies for all those items. Once we have the optimistic and pessimistic effort estimates, I then go back through the project team and ask them what is their best guess of the most likely time it takes to complete that task.
As an example, for activity A, I’ve got an optimistic time of four days, let’s say, a pessimistic time of 17 days. And then I ask them what the most likely is, and they’ll tell me something like 11.
. . . F is 16, and G is 12. Once I get the most likely time, then I can calculate the expected time for that particular task. And I use something called the beta statistical distribution to help me with that. What that means is that basically, I’ve got a weighted average to calculate the expected time required for a task. If A is the optimistic, B is the pessimistic and M, the mode, is the most likely. I use that statistical beta distribution to tell me what the actual expected time might be for that task using a formula as such, expected time is equal to A, plus four times the most likely, plus B, which is pessimistic, divided by six.
Using a spreadsheet like Excel, I can quickly calculate what that expected time might be. In this case, for activity A, the expected time with this distribution of 4 days for optimistic, 17 for pessimistic, and 11 for most likely, rounded, I still get 11 for the expected time.
. . . and 13 for G. Now, once I get my expected time for all these tasks, I can then go through my activity diagram and fill in the time required for each of these activities and determine my critical path. For example, for activity A, I’ve got 11 days. So I just go in there and write 11 days. For activity B, I’ve got 13 days, so I go and do that.
Once I fill in, the different durations for each of those activities, I can then calculate the critical path through this diagram. I can see that there’s three paths to get from starting point to end point. First off, I’ve got A-C-G path. Next, I’ve got the B-D-F path. And lastly, I’ve got the A-E-F path.
If I add up all the durations for those tasks, I get a summation such as this, 11+8+7=26. So that’s 26 days to get from A, C, and G, to that last milestone.
Now, that you’ve added up all the individual tasks and getting you through that network diagram, you can quickly see what your critical path is, which will be the longest duration it takes you to get from the starting point to the end point. In this case, it’s A-E-F of a total of 49 days.
Remember, when you’re doing activity estimation, people really aren’t that great at giving you estimates for their task. Also, there’s a tendency for people to fill the time available to complete that task. Lastly, as a project manager, if you want to have a reasonable degree of success for achieving deadlines, make sure you plan in slack in your schedule.
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