In all measured indicators the numerator is the debt service annual amount in current US dollars. Its exact "official" definition runs as follows:
Total debt service is the sum of principal repayments and interest actually paid in foreign currency, goods, or services on long-term debt, interest paid on short-term debt, and repayments (repurchases and charges) to the IMF. Data are in current U.S. dollars. Source : World Bank, Global Development Finance.
The debt service / net ODA refers to the ratio between the debt service annual amount in current US dollars and the net amount of official development aid the country receives each year. This aid, calculated as a percentage of the GNP, is defined as follows:
Aid (% of GNP): official development assistance and net official aid record the actual international transfer by the donor of financial resources or of goods or services valued at the cost to the donor, less any repayments of loan principal during the same period. Grants by official agencies of the members of the Development Assistance Committee are included, as are loans with a grant element of at least 25 percent, and technical cooperation and assistance. Aid dependency ratios are computed using values in U.S. dollars converted at official exchange rates. (Source: Development Assistance Committee of the Organisation for Economic Co-operation and Development, and World Bank and OECD GNI estimates.)
SD/ODA = Total debt service (current USD) / [(Aid (% of GNI) / 100) • GNI (current USD)].
A ratio DS/ODA = 0,457 means that the debt service represents 45,7% of the net amount of official development aid. A ratio DS/ODA = 3,487 means that the expenses due to debt service is 3,487 times more important than the net amount of official development aid.
Mode of interpretation: a rather tricky matter... The main interest of such an indicator lies in observing proportionality between debt service and aid; the "critical" value is thus 1 and shows whether the flow of debt servicing is greater than the flow of aid, and to what extent. However, since the net values of ODA are very low or even negative in some countrties and at certain times, results can be disproportionate or impossible to read when ODA value is negative. For instance, in 1983, Chile’s debt service amounted to USD 2.6 bn for hardly over USD 200,000 in net ODA, which entails a 13,000 ratio, that is, Chile’s debt service amounted to over 13,000 times the money received as ODA in 1983. Such excessive values and some negative values indicate that we have to be cautious when using these data, which are individually relevant but difficult to combine in order to assess averages in space or time.