Tobin's q

Tobin's q (or the q ratio, and Marris's v), is the ratio between a physical asset's market value and its replacement cost. It was first introduced by Robin Marris as a firm-level microeconomic variable in his 1964 book The Economic Theory of Managerial Capitalism, and was shortly afterwards further analysed by Richard Kahn in early drafts of his paper Notes on the Rate of Interest and the Growth of Firms (not published until 1972). Nicholas Kaldor in 1966, without acknowledgement to his Cambridge colleagues Marris and Kahn, repurposed the valuation ratio as a macroeconomic variable in his paper: Marginal Productivity and the Macro-Economic Theories of Distribution: Comment on Samuelson and Modigliani. It was popularised a decade later by James Tobin, who in 1970, described its two quantities as: One, the numerator, is the market valuation: the going price in the market for exchanging existing assets. The other, the denominator, is the replacement or reproduction cost: the price in the market for newly produced commodities. We believe that this ratio has considerable macroeconomic significance and usefulness, as the nexus between financial markets and markets for goods and services.