However, every activity needs to start as early as possible for the project to be completed in 15.5 months. We can use a similar process to determine which activities we can delay, and by how much, without increasing the completion time of the project. The slack represents how long we can delay the activity without delaying the entire project. The activities that have zero slack lie on a path through the network. If you delay these activities, you will delay the entire project.
Every project has at least one critical path, but there can be more than one.
The differences between these tools come from how they treat the activity time. PERT treats activity time as a random variable whereas CPM requires a single deterministic time value for each activity. We have a high degree of uncertainty in regard to the completion time of many activities. It makes sense in the real world that you do not really know how long a particular activity will take, specially talking about certain activities such as research and development. This can be possible calculating the expected or mean activity time for each activity as With the expected time for each activity we can determine which is the critical path.