the PM …[should] consult the team who would actually be doing the work on how long would it take them. It takes a little longer, but it creates a sense of ownership over the time line, because the team helped in
deciding it. Also, having team input brings to light certain assumptions and constraints which a single person might miss out on or ignore.
…Never give an estimate verbally or over the phone. Always send along a document which clearly states why you are saying it will take you x number of days…[the WBS] … , Make sure you can present your estimates
yourself to the boss or client, instead of relying on someone else to do that. The reason is that there are certain constraints and assumptions you would know better and could explain better, which would
not be lost in translation. Also, negotiations often happen at this point, and as a PM you are in a better place to know where you can cut down the time, quality or cost (the project management triangle)
because of the time and effort you’ve invested instead of the boss or a third party.
[via Green n White]