**2.2 Fractional polynomial regression of S on K**

The following Stata command returns an importance-weighted fractional polynomial regression [2, 3]:

fracpoly regress iweight POPmillions ,degre **S K**[ = ] e 9 noscaling. ( ) (1)

R2 = .9751 for this time-series regression of **S** on **K** over t = 1991 … 2018 for the entire world population. It is also important to note that this R2 is invariant with respect to the units in which **S** and **K** are calibrated; i.e. days, weeks, months, or years for survival and single, thousands, millions, billions, or trillions of current US\$ for GDP.
