Hello goye1,
You wrote:
Thanks for your answer Julie. To make it short, we have 2 projects. A 2,5 years project that is the real duration and a 2 years project that we give to the contractors (because we know there will be delays and they will finish it in 2,5 year. If we give them the 2,5 years project they will take 3 years to finish it)
So I have the 2 years project ready and know I have to prepare the 2,5 years project.
I understand your concern that the unexpected can occur in a project. However, rarely does the expected occur with such regularity that every task increases by 25%. You certainly can run the macro that PDQ kindly provided. As both PDQ and Skip have noted, this is not recommended.
My suggestion: based upon experience in managing similar projects and in working with this particular contractors, I'd extend the duration of tasks that you anticipate are likely to run long. I would however, still hold the contractor to the original dates -- or find another contractor. A 25% over run is not acceptable in my world.
As food for further thought -- is the 2 year overall duration truly achievable? Should you negotiate a more realistic duration that is somewhere between 2 years and 2.5 years?
I hope this helps.
Julie