The reason the rate of productivity growth matters so much is that it is a basic determinant of the rate of growth of average income per capita over long periods.1 To understand that one needs to know only the trick of calculating how long it takes for a growing economy to double.
A good rule of thumb for calculating the time it takes labor productivity (or anything else that is growing) to double can be calculated by dividing 70 by the growth rate. When labor productivity was growing at 3-1/4 percent per year—during the 25 years from 1948 to 1973—it took 22 years for labor productivity to double.