Critical Path Method

CPM is a schedule network analysis method frequently used in project management. The purpose is to identify one or more path that has zero or limited scheduling flexibility.
This network path is also the longest in terms of duation and is equal to the total project duration.
Once one or more crtical paths are identified, then one must compress schedule for the low risk paths and concentrate on just one critical path. The logic is that critical path activities do not have any float and thus any delay in these activities will delay the project.
Of course , it goes without saying that a non critical path which uses up all its floats can become the critical path.