So, the nodal point is — has the Verdoorn Law been validated or not in empirical studies?
There have been hundreds of studies that have tried to answer that question, and as could be imagined, the answers differ. The law has been investigated with different econometric methods (time-series, IV, OLS, ECM, cointegration, etc.). The statistical and econometric problems are enormous (especially when it comes to the question, highlighted by Romer & Romer, on the direction of causality). Given this, however, most studies on the country level do confirm that the Verdoorn law holds — United States included. Most of the studies are for the period before the subprime crisis of 2006/2007, but if anything, it is more in line with Friedman than Romer & Romer.Lars P. Syll’s Blog
Bernie Sanders and the Verdoorn law
Lars P. Syll | Professor, Malmo University