Letter from Tinbergen to Dr. C.F. Roos of the Cowles Foundation for the Advancement of Science, in response to his book ‘Dynamic Economics’. More specifically in response to the section on construction companies in St. Louis. Tinbergen holds a different opinion about the execution figures, referring to the Business Cycle Theory, and to his article ‘Ein Schiffbauzyklus?’ ( Weltwirtschaftliches Archiv , 1931). Tinbergen makes the commitment to conduct further research and to forward the results to Roos. He also asks him for a response.