Boarding house, Washington, D.C., 1942, morning bathroom line. Small slips in the bathroom schedule build up, causing unfulfilled expectations downstream, and leading to discomfort and dissatisfaction on the part of others.
Our project was in trouble. Everybody knew it. And then our project manager left the company. When our new project manager arrived, he called us all together. "I believe in taking one schedule slip," he said. Then he announced a three month slip. We all returned to work, and redoubled our efforts. It was a challenge to meet the revised schedule, but he (and we) stuck to it, and ultimately completed our development without incurring another slip.
...development is under way and progress must be tracked, avoiding major surprises to both the customer and the enterprise.
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It's difficult to know how long a project should take, and even more difficult to recover when one guesses wrong.
If you guess pessimistically, developers become complacent, and you miss market windows. If you guess optimistically, developers become burned out, and you miss market windows. Projects without schedule motivation tend to go on forever, or spend too much time polishing details that are either irrelevant or don't serve customer needs.
Therefore:
Prefer a single large slip to several small slips. ([BibRef-Brooks1995], page 24.)
"We found a good way to live by `Take no small slips' from... The Mythical Man Month. Every week, measure how close the critical path (at least) of the schedule is doing. If it's three days beyond schedule, track a 'delusion index' of three days. When the delusion index gets too ludicrous, then slip the schedule. This helps avoid churning the schedule." — Personal discussion with Paul Chisholm, June, 1994.
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This helps support a project with a flexible target date.
Dates are always difficult to estimate; DeMarco notes that one of the most serious signs of an organization in trouble is a schedule worked backward from an end date [BibRef-DeMarco1993].
A single large slip is important for the morale of the team. If you continually take small slips, nobody believes the schedule any more. This hurts morale, the sense of urgency fades, and people stop caring. On the other hand, a single large slip preserves at least some of the believability of the schedule, and people are more willing to work toward the revised schedule.
Much of the rationale is supported in the MIT project management simulation; the Borland Quattro Pro for Windows case study; and from Brooks' seminal work [BibRef-Brooks1995].
Most sane projects manage this way.
See also RecommitmentMeeting.