Dividend discount model
The dividend discount model (DDM) is a method of valuing a company based on the theory that a stock is worth the discounted sum of all of its future dividend payments. In other words, it is used to value stocks based on the net present value of the future dividends. The equation most widely used is called the Gordon growth model. It is named after Myron J. Gordon of the University of Toronto, who originally published it along with Eli Shapiro in 1956 and made reference to it in 1959; although the theoretical underpin was provided by John Burr Williams in his 1938 text "The Theory of Investment Value".
The variables are: is the current stock price. is the constant growth rate in perpetuity expected for the dividends. is the constant cost of equity for that company. is the value of the next year's dividends. There is no reason to use a calculation of next year's dividend using the current dividend and the growth rate, when management commonly disclose the future year's dividend and websites post it.
Derivation of equation 
The model sums the infinite series which gives the current price P.
Income plus capital gains equals total return 
|This section may be confusing or unclear to readers. (February 2013)|
The equation can also be understood to generate the value of a stock such that the sum of its dividend yield (income) plus its growth (capital gains) equals the investor's required total return. Consider the dividend growth rate as a proxy for the growth of earnings and by extension the stock price and capital gains. Consider the company's cost of equity capital as a proxy for the investor's required total return.
Growth cannot exceed cost of equity 
From the first equation, one might notice that cannot be negative. When growth is expected to exceed the cost of equity in the short run, then usually a two stage DDM is used:
where denotes the short-run expected growth rate, denotes the long-run growth rate, and is the period (number of years), over which the short-run growth rate is applied.
Even when g is very close to r, P approaches infinity, so the model becomes meaningless.
Some properties of the model 
a) When the growth g is zero the dividend is capitalized.
b) This equation is also used to estimate cost of capital by solving for .
Problems with the model 
a) The presumption of a steady and perpetual growth rate less than the cost of capital may not be reasonable.
b) If the stock does not currently pay a dividend, like many growth stocks, more general versions of the discounted dividend model must be used to value the stock. One common technique is to assume that the Miller-Modigliani hypothesis of dividend irrelevance is true, and therefore replace the stocks's dividend D with E earnings per share. However, this requires the use of earnings growth rather than dividend growth, which might be different.
c) The stock price resulting from the Gordon model is hyper-sensitive to the growth rate chosen.
Further reading 
- Gordon, Myron J. (1962). The Investment, Financing, and Valuation of the Corporation. Homewood, IL: R. D. Irwin.
- "Equity Discounted Cash Flow Models".