More complex arguments in ecosystem valuation regard nature's services and the assignment of values in a service economy to all that nature does "for humans". Studies compiled by Robert Costanza in the 1990s argued strongly that even just considering the most basic seventeen of these services, the combined value of the ecosystems of the earth was worth more (US$33T) each year than the whole human exchange economy (US$25T) at that time (1995). Other studies have focused on the marginal value of ecosystem changes, which can be used in cost-benefit analysis of environmental policies.