Disagreement is information#
Back of the Envelope · Week 6
Most people, in most rooms, reach for five hundred#
Averaging is a reasonable response to exactly one situation: two attempts to measure the same quantity, each carrying error that could fall either way.
That situation is rarer than the reflex suggests.
122,494 · 133,000#
ABS — Census night, August 2021
People experiencing homelessness, counted once, that night.
AIHW — financial year 2024–25
Clients of specialist homelessness services already experiencing homelessness, of those whose status was known.
One night in August 2021. One financial year, July 2024 to June 2025.#
Three and a half years apart. One is an instant; the other is twelve months.
Nothing that follows makes sense if that slips.
Five axes#
ABS
- six operational groups, a bedroom-shortfall rule
- reaches anyone counted that night
- derived from housing responses
- one night
- persons
AIHW
- people who became clients of a funded agency
- reaches only those who approached a reporting service
- assembled from records of support provided
- a financial year
- clients, linked across agencies — not visits
Definition · Coverage · Method · Timing · Unit — run down all five and most disagreements resolve into “these were never the same quantity.”
A moment against a period#
A count of who is in a condition on one night is a different kind of number from a count of everyone who passed through that condition across a year.
They can land close, far apart, or in either order — matching the label is not the same as matching the grain.
“…reflects agency engagement and does not necessarily represent the underlying prevalence…”
The AIHW, on its own client count.
Near-agreement reads as corroboration#
Occasionally close numbers really do confirm each other. More often it is coincidence between quantities that were never comparable — and this version of the error survives longest, because it never produces an argument.
Write the sentence each of them must not say#
Three people need a figure today — a funding submission, a trend claim, a staffing roster.
For each: which source answers it, and what that source will not support?
(Same two records as before — ABS’s Census night, AIHW’s funded-agency year.)