Ex-Post

What Is Ex-Post?

"Ex-post" is Latin for "from what comes after," and in finance it describes something measured or evaluated once the outcome is already known. An ex-post figure is built from actual data, such as the realized return an investment delivered over a period, rather than from predictions made beforehand. The term is often paired with ex-ante, which refers to expectations formed before the fact.

How Ex-Post Analysis Works

Ex-post analysis looks backward. An analyst takes what actually happened and measures it: how much a fund gained, how much a stock's price moved, or how closely a company's reported earnings matched earlier estimates. Because the inputs are known, ex-post results are generally more concrete than forecasts. They describe the past, however, and do not guarantee what comes next.

Ex-post measures appear throughout investing:

• Realized return, the gain or loss an investment actually produced
• Ex-post standard deviation, the historical variation of returns over a set window
• Ex-post Sharpe ratio, which compares realized returns with a risk-free rate using past volatility

An Example

Suppose an investor expected a fund to return 8% over a year, based on a forecast. At year end, the fund returned 5%. The 5% figure is the ex-post return. Comparing the two shows how well the forecast performed. A gap like this is a normal part of investing and says more about the assumptions used than about the fund's future.

Why It Matters to Investors

Ex-post figures are useful for judging past decisions and checking whether forecasting methods were reasonable. They are also the basis for backtesting, which tests a strategy against historical data. Backtests can mislead, though, if the analyst accidentally uses information that was not available at the time, a problem known as look-ahead bias. A strong ex-post record does not, by itself, predict similar future results.