The ratio between current earnings per share and share price (the EP ratio) is widely considered to be an effective gauge of under/over-valuation of a corporation's stock. Arguably, a more reliable indicator (the cyclically-adjusted EP ratio) can be obtained by replacing current earnings with a measure of 'permanent earnings', i.e. the profits that the corporation is able to earn, on average, over the medium to long run. I propose a state-space model to filter business-cycle oscillations out of current earnings and compute the cyclically-adjusted EP ratio. I estimate the model with euro-area aggregate stock market data. I find periods, notably around the 2008 financial crisis, when the adjusted and the unadjusted EP ratios provide economically and statistically different indications. I propose a method to make the adjusted EP ratio easier to interpret by translating its values into estimates of the probability that the stock market is under/over-valued. These estimates clearly indicate periods of mis-valuation in my sample. Furthermore, some simulations suggest that the model would have been able to provide early warning signs of mis-valuation in real time.
Published in 2011 in: International Finance, v. 14, 1, pp. 135-164