We've all been there: seen a report on project progress that tells that an activity, or even a whole project is n% complete. The use of the '%' of course, leds an air of calculation to the fable being peddled.
The only time I could think a percent complete would work is for some linear unit activity such as bricklaying. One could count the bricks, then work out the percent complete of the activity. But this all falls down as soon as the activity includes non-bricklaying work: cleaning the site afterwards, removing equipment, even scaffolding (now, that should be a separate activity). The estimate of progress becomes a bit of voodoo.
In a report of a major capital program that I prepared quarterly I used a different system. The two items we tracked were out-turn cost (end of project, all done and in use) and completion date (same: all done, in use). For each project in this multi $b program we tracked original approved date and cost, current approved date and cost and forecast completion date and cost.
This put each project team on the spot with clearly objective and checkable information. They were committed data!
It was reminiscent of an approach that I came across many years ago using Timeline for DOS project scheduling software.
Interesting story about Timeline. I had it at home on my old Amstrad 386 machine, and had just started working on a factory project for my firm. I don't know how the partners calculated their costs, but I put the project schedule into Timeline with all known labour costs. I took the report on project cost projection to the partner in charge who was more than surprised at the number I calculated...I don't think they had ever done that before!