This book contains a set of notes prepared by Ragnar Frisch for a lecture series that he delivered at Yale University in 1930. The lecture notes provide not only a valuable source document for the history of econometrics, but also a more systematic introduction to some of Frisch's key methodological ideas than his other works so far published in various media for the econometrics community. In particular, these notes contain a number of prescient ideas precursory to some of the most important notions developed in econometrics during the 1970s and 1980s More remarkably, Frisch demonstrated a deep understanding of what econometric or statistical analysis could achieve under the situation where there lacked known correct theoretical models. This volume has been rigorously edited and comes with an introductory essay from Olav Bjerkholt and Duo Qin placing the notes in their historical context.