They took the other countries in the G7 and said "what combination of these economies was the UK economy most like, up to 2016?"
The UK = 1/5 of Canada, 2/5 of France, 1/10 of Germany, 1/10 of Japan, 1/5 of USA, 0 of Italy.
The UK economy is really like France, a bit like Canada and the USA, hardly like Japan and Germany, nothing like Italy.
And the "Solver" add-in for Excel makes it really easy.
And so... … because I am somewhat unusual... … I present SimWales!