The act of scheduling, or calculating the schedule, refers to the process of running your Tasks, Durations, and Relationship through the Critical Path Method algorithm to identify the Starts, Finishes, Float, and Critical Path for the Project. While it's not strictly necessary to know what's going on "under the hood" of the scheduling process, a brief overview can enhance your understanding of the Critical Path Method.

Calculating a CPM schedule is a three step process. The first step is called the Forward Pass.

As you can see, this calculates the earliest date each Task can start or finish, based upon that Tasks dependencies on other Tasks, which are defined by the Relationships. These dates are called the Early Start, and Early Finish dates, and are most commonly used as the "planned" dates for a project. That's because working against the Early dates will give Tasks the largest amount of Float, in case there are delays due to weather, unforeseen conditions, or other conditions on the Project.

This step also identifies the earliest date the project can complete. The Early Finish of the last Task is the Project Completion date. This date is used as the basis for the next step of the CPM calculation, which is the Backward Pass.

This step calculates the Late Start and Late Finish for each Task, which are the latest dates a Task may start or finish without impacting the Project Completion date. If the start or finish of a Task does occur after these Late dates, then either the Project Completion will occur later, or the schedule needs to be revisited to mitigate the delay.

Once the Early Dates and Late Dates are known, the third step of the CPM process is the Float Calculation for each Task. Float is the number of days that an activity can be delayed before it will impact the completion date of the project. Activities with no float are called Critical Activities, and are part of the Critical Path.

The Critical Path identifies which activities will cause your project completion to slip if they are delayed even a single day. There are also activities which are not critical, because they have at least one day of float, but can still have an impact if they experience multiple days of delay. Activities with very small amounts of float are considered "Near Critical" and are important to watch, as they can quickly become Critical Path items if they are delayed.