A PM is responsible for coordinating disparate groups of people. So if one person says “A will take 3 days” and someone else says “I can do B in 3 days, which I

A PM is responsible for coordinating disparate groups of people. So if one person says “A will take 3 days” and someone else says “I can do B in 3 days, which I can’t start until A is done, and C in 2 days” what’s the correct time estimate for all the work? The first person will probably have issues they didn’t anticipate, so now their work is really 1 week long. The second will start on C, hopefully, then either spend a bit of time following up to see if they can start on B, or miss the fact that A is finished by a few days. So while people may be only a few days off per individual task, for a PM this has a multiplicative effect. So the PM can either act like an engineer and say “well there were X Y and Z unexpected things which is why this is going to take 3x as long” or just assume 4 “days of work” across 4-5 people really means 5 weeks before all the work is done (because not everyone can devote 100% to that work, some dependencies are difficult to hand off and can’t be done in parallel, etc).