SummaryThe basic idea of correlation analysisIn his introduction to the four following papers on applications of correlation methods, Prof. Tinbergen presents correlation analysis as the means of establishing relationships between the factors involved, in those fields of natural and social sciences, where proper experimenting is impossible.Prof. Tinbergen recommends that workers in different fields take cognizance of each other's results with applications of correlation analysis — hence this symposium. On the other hand, uncritical imitation of methods used succesfully in one field, e.g. biology, may lead to serious failure in another, e.g. economic