N Examples of Why Time Estimates are Always Wrong (Software Engineering Tips)

Client Examples
1. It costs an average of 20 minutes lost productivity every time the customer or manager phones a developer to find out how the project’s coming, in addition to the time spent on the phone
2. 4 hours are lost for each meeting that takes place between 11am and 3pm, in addition to the time spent in the meeting
3. About a week of development time is lost whenever someone from marketing makes a fuss about adding “a trivial feature that’ll only take 1 hour” so he can close a sale
4. Most clients will not provide the most important requirement until a project is two-thirds developed. They will not even realize it was a requirement until they begin to see proofs
5. Any given proof has only a 20% chance of being reviewed by the client at the time it’s submitted, and a 90% chance that the client will request changes to the proof after 1 week has already elapsed
(Link: N Examples of Why Time Estimates are Always Wrong (Software Engineering Tips))