Tobin's q theory is a ratio that compares the market value of a company to the replacement cost of its assets. A ratio below 1 implies the stock is undervalued, while above 1 means it is overvalued. This ratio is used to inform investment decisions. It is calculated by dividing a firm's market value by the book value of its assets. While Tobin's q theory was shown to predict investment in 1960-1974, later data showed the ratio and investment diverging, calling into question how accurately it forecasts capital expenditures.