Paul Collier and Anke Hoeffler (1998, p. 567) find that ‘the probability of civil war and its duration are a function of the gains from … and the costs of rebellion, made up of the opportunity costs of conflict and the cost of coordination, [based on] four proxies, namely per capita income, the natural resource endowment, population size, and the extent of ethno-linguistic fractionalisation.’ They rely on a variable of cultural distinctness, ethnolinguistic fractionalization (ELF), first used by P. Mauro (1995, p. 692) to explain the rate of growth. ELF is based on Atlas Naradov Mira (Department of Geodesy and Cartography of the State Geological Committee of the USSR 1964), ‘whose goal was to provide an extremely accurate depiction of the ethnolinguistic composition of world population’ (Mauro pp. 692, 711). For Collier and Hoeffler, all variables in the equations are significant in explaining war duration, and all variables except ELF are significant in explaining the probability of war occurring (Collier and Hoeffler 1998, pp. 568–9).