People often confuse duration and effort. Duration is the total time that it takes to complete a project measured in work days, hours or weeks. The duration depends on the availability and capacity of resources. Effort is the number of people hours needed to complete a task, i.e. it’s the actual time that is spent on working on the project. Effort can be measured in people hours, days or weeks. In order to estimate the duration of a project, the project manager will have to determine the effort first.
Let’s say you have estimated that it would take around 30 active work hours to complete building a fence, then the effort will be 30 hours. That doesn’t mean that the fence is ready IN 30 hours – unless you’re planning on building the fence for 30 hours non-stop.
For example, if you dedicate 3 hours a day to work on the fence, then the duration would be 10 days (30 hours total effort divided by 3 hours/day). But if you have a friend helping you out with the fence every day, then you would have two resources working 3 hours a day on the fence = 6 hours a day, so the duration is 5 days (30 hours total effort divided by 6 hours a day). However, if your friend can only spare 2 hours a day to help you with the fence, the duration extends to 6 days since your resources only work 5 hours a day on the fence. The total effort is always 30 hours, no matter if there is one or two people working on it. You can’t reduce the effort to 15 hours if two people work on it, but you can reduce the overall completion time.