Though they seemingly skirt over the broad truth that American businesses manufacture more than ever with less in the way of laborinputs than ever (the definition of productivity), they note that in 1950 manufacturing employed 31% of all Americans while comprising 27% of GDP.
The model begins with an input-output table, which says that, for example, the steel mill industry uses the output of the iron ore industry, the electric utility industry, as well as labor and some lesser inputs.