The process of estimating how long your project will take you and establishing project timelines reminds me of the old school strategies of drill sergeants where they test the ability of their new enlistee to perform under pressure by asking questions that have no right answer and giving orders that conflict. Accurately estimating the time requirements and constraints of your project as part of the planning process is extremely important. While you may feel that this process is difficult and there may be no right answers, professional project managers rely on what are called three-point estimates to help guide them through the process.
Using the expertise of key stakeholders and team members you may have already identified as well as the three-point process, your project calendar, budget, and the requirements on the time of your team members will ultimately be more realistic. At this point of the planning process you should have already completed your work breakdown structure (WBS), defined the activities supporting the WBS and grouped them into work packages, sequenced your activities, and finally determine what personnel and other resources you will have at your disposal. If you mapped the critical path for the WBS and supporting work packages with the dependencies for each task already detailed, you are ready to proceed.
In each of the work packages on your Gantt chart, you need to determine three timeframes: the most likely time of completion, the optimistic estimate, and the pessimistic estimate. I suggest you make a decision early in the process to either use hours or days as your unit of measure, but stay consistent. For each work package, you should note the three numbers. If you are using software like Microsoft Project, this should be easy. The difficult part of this step is nailing down these three numbers accurately. You really need the input of your team or an expert if this work package is outside of your area of expertise.
After you have recorded the timeframes for each of the work packages, you will then follow the lines for each dependency on the Gantt chart to determine the length of time required for the project. You should add the times for each category together, which will give you three numbers that correspond to the categories I outlined. There is a great tool used by the best project managers called the Program Evaluation and Review Technique (PERT) to help you accurately estimate how long it should take to complete your project.
Most project management software will automatically estimate the time to completion for you, but let’s manually figure it out so you understand. Your input will be the sum of the total time for the three categories (most likely, optimistic, and pessimistic). In the example, let’s assume your most likely was 50 days, your optimistic was 40 days, and the pessimistic was 60 days. The PERT formula to provide you with a weighted estimate of the total project time looks like this:
(Optimistic + (Most Likely * 4) + Pessimistic) / 6
In our example, plugging our numbers into the fomula would look like this:
(40 + (50 * 4) + 60) / 6 = 50 days
I used numbers that would be simple, but you can see where this process would be helpful. From this point you will take the results of this process and begin to determine the schedule for the project implementation. Like with every step of the planning process, take time and put some effort into this step. The more accurate your estimates are, the better your ability to provide the defined deliverables on time and within budget.