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).