 The ways in which models function in economic measurement have led some philosophers to view certain economic models as measuring instruments in their own right, analogously to rulers and balances (Boumans 1999, 2005c, 2006, 2007a, 2009, 2012a, 2015; Morgan 2001). Marcel Boumans explains how macroeconomists are able to isolate a variable of interest from external influences by tuning parameters in a model of the macroeconomic system. This technique frees economists from the impossible task of controlling the actual system. As Boumans argues, macroeconomic models function as measuring instruments insofar as they produce invariant relations between inputs (indications) and outputs (outcomes), and insofar as this invariance can be tested by calibration against known and stable facts. When such model-based procedures are combined with expert judgment, they can produce reliable measurements of economic phenomena even outside controlled laboratory settings (Boumans 2015: Chap. 5).