Missing work is more common than bad arithmetic
Many late estimates are not caused by poor multiplication. They happen because part of the work was never represented: setup, access, procurement, review, cleanup, rework, documentation or coordination. When those pieces appear later, the schedule seems to “slip” even though they were always necessary.
A short pre-task checklist is often more useful than adding a large arbitrary contingency. Ask what must be ready before work begins, who must approve it, what happens after the main task, and whether the work blocks or depends on something else.
Optimism and the single-number trap
People naturally imagine a clean run: the material is available, the file opens, the decision-maker responds and nothing has to be repeated. That best-case mental model is useful for understanding the work but weak as a commitment.
Use an expected case and name the uncertainty. If the unknown is an external response, model it as waiting time. If the task itself varies, use historical percentiles or a range once enough data exists.
Interruptions and utilization
A person can be scheduled for eight hours without having eight hours of uninterrupted productive time. Meetings, messages, switching between jobs and ordinary coordination consume capacity. The Time Budget Calculator lets you apply a focus or utilization factor rather than assuming every available hour is usable.
High utilization can also increase queues. When every person or machine is booked tightly, a small delay has nowhere to go and can push several later tasks. A little visible headroom can improve reliability even if it looks less “efficient” on paper.
Learning from variance
Do not merely add more buffer after a late job. Classify the cause. If estimates are consistently low by the same proportion, update the rate. If one step is repeatedly missing, add it to the template. If external waits dominate, work on the handoff or service level rather than the labour estimate.