Table 2. International collaboration by economic groups of countries (1996–2012).

Collaborating countries (1)	Collaborating countries (2)	Frequency	Average number of authors	Average number of institutions	Average number of countries	Average study sample size
High-income	Middle-income	49 (50%)	8.7	3.9	2.3	628
High-income	Low-income	36 (36.7%)	9.5	4.6	2.8	637
High-income	High-income	11 (11.3%)	9.6	5	2.5	186
Low-income	Low-income	2 (2%)	8.5	2.5	2.5	216

International collaborations were considered when the paper involved different countries, 98 in total. The combination of countries according to economic groups considered income extremes. For example, if there was a collaboration involving one high-income country and two middle-income countries, this was classified as a high- and middle-income country collaboration, not as middle and middle.