For most countries, data on GDP were not available before 1914, and therefore, proxy
variables, such as Gross National Product (GNP) or Net National Product (NNP), were used
for computing debt ratios in the earlier years. From 1914 through 1980, GDP data for most
countries were mainly sourced from Mitchell (2003).
Government publications were also
used for collecting GDP data for some countries, including Canada, Italy, Japan, the
Netherlands, New Zealand, the United Kingdom, and the United States.