In such cases, a schedule "is obtained by estimating the duration of each task and noting any dependencies amongst those tasks". Dependencies, in turn, are tasks that must be completed in order to make other tasks possible, such as renting a truck before loading materials on the truck (since nothing can be loaded until the truck is available for things to be loaded on). Scheduling of projects, therefore, requires the identification of all of the tasks necessary to complete the project, and the earliest time at which each task can be completed. In creating a schedule, a certain amount of time is usually set aside as a contingency against unforeseen days. This time is called scheduling variance, or float, and is a core concept for the critical path method.