Schedule Padding

Kirk: "Mister Scott. Why do you multiply your repair estimates by a factor of four?" Scotty: "To preserve me reputation as a miracle worker, sir!" -- Star Trek V: The Final Frontier

Additional time added to a schedule, either as a whole or to particular tasks, to increas the chance that the schedule will be met. Often this is legitimate, sometimes it is not.

Legitimate reasons:

As a way of accounting for unforseen and unmanageable risks/circumstances--i.e. the lead developer getting hit by a bus, a BugFromHell, etc. These should be added to the schedule as a separate task, occurring at the end, and the number should be (reasonably) justifiable.
As a way of accounting for known risks, such as a notoriously difficult task where a rock-solid estimate is not possible.
To account for the normal workday distractions (meetings, etc.) Individual tasks should have take the normal work routine into account, rather than being "if I were chained to my desk" estimates. (Don't put separate tasks in the schedule for this; else you'll go crazy).

Illegimate reasons:

To make it virtually impossible for the task/project to slip (pad should be used to make meeting schedule a likely scenario, not an absolute certainty). See ScheduleBlackjack.
To give time to loaf. (Should be obvious).

Also, one other note:

Sometimes, different levels of management (including developers, though not really "management") each add their own amounts of pad to the schedule, assuming that their subordinates are producing estimates too aggressive. This redundant pad can accumulate, making a project appear unattractive. All padding and assumptions should be stated.

See also ScheduleCorrectionFactor

A probability estimation curve would be the best way to present time estimates in my opinion, but many in management won't relate to them. Example:

2 weeks: * 3 weeks: **** 4 weeks: ****************** 5 weeks: *************** 6 weeks: ******* 7 weeks: ** 8 weeks: *

Under this distribution, I'd tell them "about 6 weeks", typically roughly around the 75% point.

--top

The railway timetables in the U.K. do this as a matter of routine practice. Trains are timed along a journey and are checked at the main arrival points for punctuality. The internal working timetables are different from the published timetables. The working timetables show extra time called recovery time just before the arrival at the main destination, to make it more likely the trains will be on time. The percentage of on time trains is an important measure for the train companies. -- JohnFletcher