Congresses 255 Economic aspects 193 Mathematical models 188 Licenses 176 Radio stations 135 Testing 127 Research 123 Data processing 114 Design and construction 96 Economic conditions 87 Roads 82 Statistics 80 Measurement 79 Environmental aspects 76 Education 67 Geology 64 Agriculture 58 Radio broadcasting 56 Accounting 55 Planning 55 Bibliography 54 Management 54 Transportation 53 Study and teaching 52 Evaluation 50 City planning 49 Pollution 49 Information storage and retrieval systems 47 Periodicals 47 Safety measures 46