The sources are: OECD, 2017a, Bhattachaiya et al., As a proportion of global GDP, these figures are in the range 4.8 per cent to 8.3 per cent, assuming global GDP grows in real terms over the years 2016-2023 according to projected rates of IMF WEO Database April 2018 and then at 3 per cent over 2024-2030. These proportions might be compared against investment estimates presented by Woetzel et al. ( Second, that it is economically sustainable, whereby it creates jobs and boosts growth but does not create unsustainable debt burdens for the government or high costs for users.