The first hundred days spend the only period in which changing the arrangement costs almost nothing politically.
Why it matters when the plan changes
Two things are abundant immediately after a change of ownership and scarce afterwards: permission to change how things work, and the assumption that things will change. Both decay quickly. New ownership is one of the events that changes what a plan can reasonably ask of an organisation, which is why the reading has to be taken then, not at the next annual cycle. A structural question left open past the first hundred days becomes a negotiation rather than a decision, because by then someone has already started behaving as though they own it.
The tension is that the period is also when the new owner knows least. Acting early buys permission and risks acting on a misreading; acting late buys information and spends the permission. The compromise that usually works is to settle ownership and sequencing early and leave substantive strategy until the evidence is in.
In practice
A new owner uses the opening period for meetings and diagnostics and defers structural decisions until it understands the business. By month five the informal answer to who owns pricing has settled, twice, in two different regions. Correcting it now removes something from someone, which in the first month would have been an appointment.
Evidence
A change of ownership is one of the events that changes what a plan asks of an organisation, which is why the reading has to be taken then rather than at the next cycle.
Donald Sull, Rebecca Homkes and Charles Sull, Why Strategy Execution Unravels and What to Do About It, Harvard Business Review (2015)
What it cannot tell you
A 100-day plan cannot tell you whether the initial reading of the business was correct; it fixes direction before the evidence is complete, and a plan settled early on a misdiagnosis becomes as hard to unwind as one left open too long. It says nothing about the quality of the read itself.
Questions
Decisions that are cheap now and expensive later: who owns which call, how the reporting and forums work, what sequence the early moves run in, and which questions are deliberately being deferred. Substantive strategy usually belongs after the evidence is in, not inside the window.
The number itself is a convention, not a finding; work by Donald Sull, Rebecca Homkes and Charles Sull in Harvard Business Review (2015) shows how quickly execution slips once conditions shift. What is real is that permission decays and informal ownership settles fast, so the window lasts only as long as those conditions hold.
Using the window entirely for learning and deferring every structural decision. The learning is valuable and the deferral is expensive, because ownership does not wait to be assigned. It settles informally, and unwinding it later costs far more than assigning it would have.
The parts that tell people what they must decide differently, yes. A plan held by the owner and the top team leaves everyone else interpreting the change locally, which produces several private versions of the new logic, each defended in good faith by someone acting reasonably.
It is the opening sequence of it. Research reported in Harvard Business Review (2015) found only 9% of managers say they can rely on colleagues in other functions and units all the time, which is why unassigned dependencies quietly narrow the later plan's options rather than remaining neutral.