Politics and government 20 History 19 Education 14 Mathematical models 12 Climatology 8 Research 8 Economic conditions 7 Atmospheric circulation 5 Charts, diagrams, etc 5 Description and travel 5 Economics 5 Political parties 5 Political psychology 5 Political science 5 Statistics 5 Atlases 4 Case studies 4 Economic policy 4 Education, Secondary 4 Methodology 4 Monetary policy 4 Money 4 Public administration 4 Science 4 Social conditions 4 Air 3 Civilization 3 Curricula 3 Data processing 3 Econometrics 3