If you read my Guide to Network Analysis, made available last month, you hopefully gained a better understanding of a project’s “Network” and “Analysis.” The “Network” part being a logic diagram of connecting nodes (details of the individual tasks and the relationships between them), and the “Analysis” part describing the process of estimating the duration of each task and then calculating the timings for each task.
You may say that duration depends primarily on the quality of the resource doing the work, external factors like the weather, and so on. However, I believe the duration calculation needs a more hands on approach than that. A correct analysis of duration will act as the baseline for your project and providing you with an end date. It would be wise to generally restrict your deliberations to the time ONE resource will take to complete each task.
The only modifier you might have to use is for a task that, by its nature, will require two or more resources (or even a team). Refer to my example of carrying a long plank. If that is the case, base the duration calculation on one team’s effort. Again, I have a warning! Be aware that if you assign a team to do the work, then an individual resource within that team might have to be treated as a different resource for other tasks that may overlap.
The duration should be determined by the best qualified person using estimation techniques, previous knowledge, and/or even statistics! Try asking the resource that is going to do the task. You might get some interesting data!
I cannot stress enough the best practice of using a single resource duration, as that will end up giving you the best realistic figure for the end date. It will also allow you, in the future, to add additional resources reducing slippage of the overall time—time you would not have available otherwise.
Stay tuned! Next time, I’m planning to cover the forward and backward passes.