Column
Why estimates slip
Estimates tend to slip one way, late, and the reasons are well known. This column reads from the estimate itself, to how work behaves once it starts and while it waits, to what a late project tempts a team to do, and then to planning that expects the slip.
It starts with the estimate, which is made when the least is known.
Hofstadter's Law
Work takes longer than estimated, even when the estimate already allows for that.Keep in mind: Trust what past work actually took over how this work looks todayThe gap is hard to see early on, because early progress is easy to count.
Ninety-Ninety Rule
The last 10% of the work tends to take as long as the first 90% did.Keep in mind: Expect "almost done" to hide as much work againYou might hope tasks that come in under would offset the ones that run over. Tasks that could finish early tend to expand to fill their time, so early finishes may not offset the late ones.
Parkinson's Law
Work expands to fill the time available for it.Keep in mind: Give open-ended work a short boxOne delay sits outside every task's estimate: by the same arithmetic as for servers, a team whose every hour is booked can expect new work to wait in line before it even starts.
Kingman's Formula
As a server's utilization approaches 100%, the time work spends waiting for it climbs steeply, and variability in arrivals and service multiplies the climb.Keep in mind: Keep headroom wherever someone is waiting on the answerWhen the date is at risk, the obvious fix is more people.
Brooks' Law
Adding people to a late project tends to make it later, because ramp-up and coordination cost more than they add.Keep in mind: When a project is late, cut scope or move the date before adding peopleBesides adding people, a late project can move the date or the scope. Moving the date is padding by another name, and padding tends to get used up. Moving scope is the one that can be planned in from the start.
Fixed Time, Variable Scope
When scope is fixed and time is not, the date slips and the scope stays; fixing the time instead forces a choice about what matters.Keep in mind: Decide what the work is worth before you ask how long it takesMoving scope means deciding what ships inside the box and what waits, and what waits often keeps losing value.
Cost of Delay
Work gets ordered by size, by gut feel or by whoever asks loudest, while what each item loses every week it waits goes uncounted, so the most valuable work often waits behind the merely visible.Keep in mind: Ask what a week of waiting costs before asking what it costs to build
