也要考慮interest rate on national debt, 最後是看國債總利息相對於GDP的比例。

https://fred.stlouisfed.org/series/FYOIGDA188S#:~:text=Table_title:%20Federal%20Outlays:%20Interest%20as%20Percent%20of,%7C%202025::%202021:%20%7C%203.15344:%201.48505%20%7C