Verify Your Estimates
In most cases, unless you are very familiar with the task at hand, you should check with whoever is going to do the work to find out how long it will take them to complete it. If the time frame sounds unreasonable, research how long it takes someone else to do the same task. For example, if a printer says he can have the job in five weeks but you feel, based on your experience, that it should not take that long, check with a couple of other printers to determine their turnaround time. If two other places can have it done in three weeks, clarify why your printer cannot do it in that same time period. Often, team members or outside vendors like to pad their own schedule in case they have to push the work back a few hours or a few days. This is fine if you have some flexibility in your schedule. However, that isn't always the case, especially in business.
Once you've set up the time frames in your network diagrams, go back and reconfirm that they are okay. Suppose you accepted the printer's estimate that he could do the job in four weeks because you like his work and have that much time available in the schedule. You still need to confirm that he indeed understands that you have it on your schedule to be completed in four weeks, and have that due date clear on both of your calendars.
Estimating the Unknown
Many times, you will be asked to estimate a task and will have no idea of what it will take. In this case, follow these steps:
Break down the task into individual steps
Come up with Best Case, Most Likely, and Worst Case time estimates for each step
Add up each column and apply the following formula:
1 Best Case + 4 × Most Likely + 1 Worst Case)/6
The formula (BC + 4×ML + WC)/6 is called a PERT formula (Program Evaluation and Review Technique). This formula was started in the engineering world and has been widely applied to project management to estimate the unknown. There are many more formulas within PERT, but most project managers stop here. Even with this formula it is difficult to get the right answer, but it has helped many a project manager create a better estimate when the work effort is truly unknown.