Measure the product before measuring productivity.

across the board execs complaining about productivity turn out to be poor at defining product ("its just a website, how long could it take to build, Jeez").

Any productivity comparisons between software and other manufacturing processes should begin with a few minutes spent to compare software specs and the said product's spec, see how hard it is to change its spec ("add a button to accept payments" v/s. "add a knob on the car's dashboard")

provide a technical spec first, then we can talk about productivity.