For example, it has been pointed out that debtors with a high propensity to consume tend to restrain their spending when the real value of their debt rises, weighing down the economy. In addition, due to the zero bound on nominal interest rates, real interest rates do not fall to the level required by the level of economic activity.
Usually, total factor productivity is used as the key measure. It is determined as a residual: productivity growth is the increase in value added that cannot be explained by the volume of inputs used. This measure is very rough. All in all, potential output and its growth rate is more difficult to determine than it was in an industrial economy.