Each workstream is managed separately and most of the risk lives in the joins between them.
Why it matters when the plan changes
Programmes are decomposed into workstreams because that makes them manageable, and the decomposition creates the interfaces where execution actually fails. Each stream is tracked, resourced and reported on its own, while the dependencies between them belong to nobody and appear only as an escalation when one slips. Sull, Homkes and Sull's 2015 Harvard Business Review research found that only 9% of managers say they can rely on colleagues in other functions and units all the time, the exact commitment a workstream boundary demands.
The tension is that reporting by workstream is genuinely useful and structurally incomplete. A view that shows eight streams and their status cannot show that three depend on the same manager, or that two hold incompatible assumptions about sequencing. That only surfaces if someone holds the whole and is asked to look. Brooks's law holds that communication channels multiply faster than the number of parties, so each added workstream costs more to integrate than the last.
In practice
A programme reports eight workstreams, all green. Three of them assume the same infrastructure change lands in the second quarter, and the stream delivering it has assumed a third quarter date since January. Each status report is accurate. Nothing in the format compares the assumptions.
Evidence
Cross-unit commitments are the least reliable part of execution, and workstream boundaries create them by design.
Donald Sull, Rebecca Homkes and Charles Sull, Why Strategy Execution Unravels and What to Do About It, Harvard Business Review (2015)Communication paths grow faster than the number of parties, so each additional workstream costs more to integrate than the last.
Brooks's law, after Frederick P. Brooks, The Mythical Man-Month (1975)
What it cannot tell you
Workstream execution risk locates where exposure concentrates, it does not measure how likely a given dependency is to fail or how severe the consequence would be. Naming the interface between two streams does not tell you which side's assumption is wrong, or resolve who should own the fix; that judgement still requires someone to look.
Questions
Because the decomposition that makes a programme manageable creates interfaces, and each interface is a commitment between parties who do not report to each other. Sull, Homkes and Sull's 2015 Harvard Business Review study found only 9% of managers can rely on colleagues in other functions all the time, and a workstream boundary is that same commitment.
Shared assumptions that conflict, shared resources that are over-committed, and sequencing dependencies where each side has assumed a different date. Every individual report can be accurate while the programme is carrying a contradiction that only appears when someone compares them.
By listing the dependencies between streams explicitly, with a named owner on each side and an agreed date, and reviewing that list rather than only the stream statuses. It is a short list and it is usually the most informative document in a programme.
One named person, chosen deliberately, usually on the receiving side since they are the ones who carry the consequence of it slipping. Leaving it jointly owned reproduces the original problem, and escalating each occurrence produces a coordination office reporting the same blocker every month.
Yes. Brooks's law, drawn from Frederick Brooks's 1975 The Mythical Man-Month, holds that communication channels increase faster than the number of parties. Applied to programmes, a programme of twelve streams carries far more than twice the coordination exposure of one with six, because each addition multiplies interfaces rather than adding one.