The nominal value of the debt is not a good measure because it disregards the fact that the value of a single unit (the dollar) changes over time, and doesn't r

The nominal value of the debt is not a good measure because it disregards the fact that the value of a single unit (the dollar) changes over time, and doesn't reflect the difficulty in paying it off (i.e. % of national GDP). It's also a bit of apples-and-oranges when you compare the nominal debt of one country with another without looking at the differences in GDP between the countries, i.e. the size of each country's economy.