Often ideas don’t work as well in practice as theory would suggest. In the investment world that is often due to differences between how investment ideas are formulated in theory and how they are put into practice. Many rules-based investment strategies claim heritage to independent academic research demonstrating that certain factors like value, profitability, momentum, and small size have been associated with higher expected returns. But there are critical differ­ences between the way academic researchers have documented those factors and how investment managers design actual strategies. These adjustments are often necessary to mitigate transaction costs and increase capacity, but they can prevent a strategy from fully capturing the returns shown in the academic literature.

A review of the commonly accepted formulation of the value factor that Eugene Fama and Ken French popularized in their 1992 paper, “The Cross-Section of Expected Stock Returns,” will help illuminate these differences. In deriving their version of the value factor, the pair begins with a universe representing all stocks listed on the New York Stock Exchange, Nasdaq, and American Stock Exchange. Stocks with market capitalizations lower than the median stock on the NYSE go into the small-cap group, while those above this threshold go into the large-cap group. Fama and French rank the stocks in each size group once a year in June by their book/price ratios at the end of December. Those in the top 30% (by count) are allo­cated to the small- and large-value buckets, while the bottom 30% go into the growth buckets, as Exhibit 1 illustrates. Stocks in each bucket are weighted by market capitalization. The value factor is calculated as the average return on the two value portfolios minus the average return on the two growth portfolios.