… is from page 161 of Fritz Machlup‘s 1964 collection International Payments, Debts, and Gold; in particular, it’s from Machlup’s chapter – written exclusively for inclusion in this volume – “The Mysterious Numbers Game of Balance-of-Payments Statistics”:
[T]here is insufficient realization of, or at least insufficient emphasis on, the impossibility of obtaining reliable and permanently relevant statistical counterparts for the theoretically important concepts.
See also this superb 1991 essay by Bob Higgs, in which he details problems with data collection and interpretation that are too often overlooked.
