The assumption you didn’t notice you made

Back of the Envelope · Week 3

£3.5 million, 1957

The estimate for the Sydney Opera House.

Completed in 1973, at approximately A$102 million.

Do not divide one by the other

  • currency change — Australia decimalised in 1966
  • sixteen years of inflation
  • a client decision to expand the number of performance spaces
  • Jørn Utzon’s 1966 departure and a different design for the interiors

A ratio across all of that is a precise number about nothing.

“…hadn’t been properly surveyed when competition guidelines were written.”

Bennelong Point’s geology, from the Opera House’s own published history of construction.

Ranked by likelihood

(one way to rank these by likelihood of being wrong)

  1. The roof’s load can be settled once construction starts
  2. The design will be finalised before it matters
  3. The preparatory site work is already covered by the estimate
  4. Bennelong Point’s ground will support the design

Ranked by consequence

(the same four assumptions, ranked by how far the answer actually moved)

  1. The preparatory site work is already covered by the estimate
  2. Bennelong Point’s ground will support the design
  3. The roof’s load can be settled once construction starts
  4. The design will be finalised before it matters

Checkable in 1957, or unresolved in 1957

Checkable at the time — the ground: a survey could have settled it before the guidelines were even written.

Genuinely unresolved — the roof load and the final design: not withheld, not yet decided by anyone.

Name the one link in your Week 2 chain you’d least like to defend.

Then ask: could you have checked it at the time — or was it genuinely still open?

← Back to lecture