What Is Alpha?

In trading and investing, alpha means returns that cannot be explained by exposure to the market or to well known risk factors. If a portfolio earns 12% when its market exposure alone would have produced 9%, the 3% difference is, loosely, alpha. Alpha is what active traders, hedge funds and quant firms are trying to find. It is also scarce: most apparent alpha turns out to be hidden market exposure, factor tilts, luck or a backtest artefact.

Alpha vs beta#

Beta is cheap: index funds deliver market exposure for very low fees. Alpha is valuable because it is hard to get and adds return that does not simply come from taking more market risk.

Measuring alpha#

The simplest approach regresses a strategy's excess returns on the market's excess returns:

R_p - R_f = α + β × (R_m - R_f) + ε

The intercept α is the alpha. Using more factors, such as size, value, momentum and quality, gives a stricter test. See Factor Models and Regression Analysis.

Sources of genuine alpha#

Alpha, luck and statistics#

Alpha estimates have large standard errors. A 2% annual alpha with 10% tracking error needs many years of data to be statistically significant. Over short periods, luck dominates. Studies of mutual funds, such as Fama and French (2010), found that after costs, very few managers showed evidence of skill beyond what luck would produce. See Statistical Significance in Trading.

information ratio = alpha / tracking error
t statistic ≈ information ratio × √years

Alpha is zero sum before costs#

In aggregate, all investors together hold the market. Before costs, the average active dollar earns the market return, so one investor's positive alpha must be matched by another's negative alpha. After costs, the average active investor underperforms. This arithmetic, described by William Sharpe in 1991, is why consistent alpha is rare. See Active vs Passive Investing.

Alpha decays#

Once an edge is discovered and exploited by others, it tends to shrink. Strategies must keep evolving. See Signal and Alpha Decay.

Alpha for individual traders#

For an individual trader, a simple test is to compare your results with a cheap alternative of similar risk, such as an index fund held at the same average exposure. If your returns after costs do not beat that alternative over a long period, the effort may not be adding value. Tracking this comparison honestly in your journal is one of the most useful habits in trading. See Trading Journal.

Frequently asked questions#

What is alpha in trading?#

The return a strategy earns beyond what can be explained by its exposure to the market and other known risk factors.

What is the difference between alpha and beta?#

Beta is return from market exposure; alpha is return that remains after accounting for that exposure.

Is alpha easy to find?#

No. Most apparent alpha is explained by hidden factor exposure, luck or backtest errors, and true alpha tends to decay as others find it.

Next, learn how to look for new sources of alpha in Signal Discovery.