Unhappy with late deliveries, a project manager decides he can reduce variability by inserting a safety margin or buffer in his schedule. He reduces uncertainty in the schedule by committing to an 80 percent confidence schedule. But, what is the cost of this buffer? The project manager is actually trading cycle time for variability. We can only know if this is a good trade-off if we quantify both the value of cycle time and the economic benefit of reduced variability.
Donald G Reinertsen in The Principles of Product Development Flow