If you are an inexperienced project manager you've probably gone down the "percent complete abyss" at some point.
Here's how it goes: The project manager assigns a piece of work to a team member and asks that it be done by Friday. The team member agrees but sure enough, on Friday the work is not done. The project manager asks how close it is and the person responds "90% complete." The project manager walks away, confident the work is almost done. The next Tuesday the project manager asks if the work is done and the person says no, but he is 95% done. The project manager is satisfied that progress is being made and walks away. On Friday the work is still not done, but the team member is now 99% done. The next Tuesday he is 99.9% done.
You get the picture. If you're the project manager, what have you learned when a person says he is 90%, 95% or 99% done? In fact you haven't really learned a thing. When a person tells you that an activity is 90% complete, what they heck does that mean? Why not 85% complete or 93% complete? It's all just a subjective guess.
(I understand that percent complete may have value on a project basis if you're using earned value calculations. But it's meaningless on individual activities. Even in earned value, you usually care only about precision with three values -- 0% -- not started, 50% -- started, 100% -- completed.)
The better way to manage the work is to ask "when will the work be done?" If the schedule shows an activity should be completed on Friday and the work is not done, don't ask the team member for the percentage complete. Instead ask the team member "when will the work be done?" Asking when the work will be completed gives you concrete information you can place on your workplan, while also getting the team member to make another commitment to the new end date.
Here's how that scenario plays out in the prior example with our team member named Joe.
Project manager: "Joe, your assignment was due today and I see you're not done. Let's discuss why." (They discuss the reasons.)
Project manager: "When will the work be done? Next Tuesday. Okay."
(Next Tuesday arrives. Work is still not done.)
Project manager: "Joe, this work was due last Friday. Then you told me it would be done Tuesday. The work is still not done. Why?" (They discuss the reasons.)
Project manager: "When will the work be done?"
Again, you see how the scenario plays out. Asking for when the work will be done requires Joe to re-commit. After missing two deadlines, hopefully Joe now feels a genuine obligation to get the work done -- a level of commitment that doesn't exist in the 90%, 95%, 99% scenario. This also gives the project manager information in terms of Joe’s ability to manage his time, estimate his work correctly and meet his commitments.
Managing by due date is the only way to proactively manage the project workplan.