When you build a schedule you need to understand how to estimate duration. If everyone worked eight hours per day, and was 100% productive for all eight hours, you could easily calculate duration by taking the number of effort hours, divided by the number of resources. For instance, if an activity was estimated at 80 hours, and you have one person assigned, and he works eight hours per day, the duration would be (80 / 8) = 10 days. Likewise, if two people were assigned full time, the duration would be (80 / 2 / 8) = 5 days.
Estimate the productive hours per day Normally the first step is to determine how many productive hours of work you can count on each person working per day over time. Using a factor of 6.5 productive hours per day will help you take into account socializing, ramp-up time, going to the bathroom etc.
Determine how many resources will be applied to each activity In general, the more resources you can apply to activities, the quicker the activities can be completed. (These are called resource constrained activities.) You need to estimate how much duration can be saved with additional resources. Obviously two resources may be able to complete an activity faster than one person, but it may not be twice as fast. Similarly, a third person may allow the task to be completed sooner, but not in one-third the time. Each additional resource may shorten the duration incrementally - up to a point where additional resources actually will result in a longer duration.
Factor in available workdays Take into account holidays, vacations and training. This was not included in the productivity factor in the first item, since this non-project time can be scheduled and accounted for in advance. For instance, on a three-month project, one team member may be out for two vacation days, while another may also have ten days of vacation. To make your schedule more accurate, take into account any days that you know your team will not be available to work on the project.
Take into account any resources that are not full-time actor in any resources that are not full time. For instance, if you have a resource allocated 50% of his time, it will take at least twice as long to do any individual activity. If you have an activity that has an estimated effort of 40 hours, and you assign a resource that is only allocated 25% to your project, the resulting duration will be at least four weeks, if not more.
Calculate delays and lag-times Some activities have a small number of effort hours, but a long duration. For instance, a deliverable approval may take one hour, but might take two weeks to schedule the meeting. You need to take this lag time into account for your estimated duration.
Identify resource constraints When you build your initial schedule, you identify the activities that can be done sequentially and those that can be done in parallel. If you have enough resources, all of the parallel activities can, in fact, be done in parallel. However, you can only do the activities in parallel if you have the right resources available at the right time. There may be a set of activities that can be done in parallel; however they need to be worked on sequentially because only one person has the right skills to do the work. Be sure to factor in resource constraints. This adds additional duration.
Document all assumptions You will never know all the details of a project. Therefore, it is important to document all the assumptions you are making along with the estimate. by TenSteps