The metric is calculated by dividing an index’s inflation-adjusted price by the average of its inflation-adjusted annual earnings over the last 10 years.
But how does one define “earnings”? As far as the metric is concerned, the answer doesn’t matter, as long as the definition is consistent across time. If the definition is consistent across time, then apples-to-apples comparisons can be made between the metric’s present value and its prior values. The comparisons will give an accurate indication of how cheap or expensive the index is relative to its history, or to what is “normal” for it.
Unfortunately, the earnings
data on Dr. Shiller’s website, which are used to build the Shiller CAPE, are
not based on a consistent definition of “earnings” across time. The data are taken from S&P “reported” earnings, which are formulated in accordance with Generally Accepted Accounting Principles (GAAP). But the standards of GAAP have changed significantly over the last few decades.