Cushion Time in Project Planning
Inserting cushion times into a project schedule during project planning can improve your project management, but only if you follow some basic procedures. You have to decide whether and how to insert cushion times for the best result. Some ways of including cushion time don't delay the scheduled finish of the project, and you have to avoid causing unnecessary delays by inserting too much cushion time.
The schedule of your project plan shows different activities, their start dates and projected completion dates. The activities are made up of tasks with estimated durations. When you're not sure whether a task can be completed in the allocated time, you can add an extra cushion and lengthen the total duration. The extra task duration delays the completion date of the activity and may delay the project finish date, but you are now more certain that the task in question can be completed in the estimated time.
When you schedule your tasks, you usually find that some tasks can't start because they depend on other tasks to finish. This results in time cushions between tasks. For example, you may want to schedule laying sod right after the spreading of topsoil but can't because the sprinkler system isn't connected and the plumber's schedule is full until the following week. The time cushion in the schedule before you can start to lay sod is called slack time, and you can use it to optimize the project schedule. You can reduce work on the tasks that have slack time and assign more workers to the task that is holding you up. In this case, you may be able to take one of the workers spreading topsoil to help the plumber finish earlier.
Task contingencies are specific time cushions you assign to risky tasks. The process differs from increasing the duration of a task with a cushion, because the contingency amounts remain separate from the tasks. If you don't know how long a task might take, you can assign a time cushion as a contingency to make sure you can complete the task by the projected finish date. If you later get new information that lets you estimate the duration of the task accurately, you can reduce or remove the contingency. Since the contingency time cushion remains a separate amount, it is a more flexible approach than increasing task duration.
If you are uncertain how long an activity will take overall, you can insert time cushions as buffers between the tasks of the activity. Buffers remain explicit time cushions separate from individual tasks and disappear when the task following the buffer starts. The start date of the task reflects the actual start, and the buffer assigned before that date becomes meaningless. You may have used some of the allocated buffer time to start the task earlier than originally scheduled, and you may be able to reassign additional buffer time later in the project if risks there have increased.