[snorberhuis]

Good point you hardly ever start with a clean slate. Using historical data or asking engineers to estimate how long it will take will always be based upon this past performance.

But the point I try to make is that it is hard to take into account all the factors you have to deal with in a complex situation. As a human, you tend to ignore irregular influences. With tracking tools, you get this data right out of the box and it is more accurate in my opinion.