2013issue C0112-17
Classroom books, leftover beta, and diversification
A classroom portfolio exercise treats total return variability as firm-specific unsystematic risk plus market-related systematic risk. Adding names from less related industries reduces the firm-specific piece until a residual remains, and that leftover exposure is what the CAPM prices with beta.
- Total return variability is presented as the sum of firm-specific unsystematic risk and market-related systematic risk.
- Adding securities reduces total portfolio variability until a market-related residual remains, with the decline described as leveling off after roughly 10 to 35 names.
- Under the CAPM framing, the relevant risk of a security is its systematic exposure, and the expected-return premium is beta times the market excess return.
- A five-month student exercise held other variables constant across two five-name, five-industry books; the lower-beta book lost less, which the source treats as minimized risk rather than proof that unsystematic risk had been fully removed.
Two kinds of return variability
The archive presents total return variability as the sum of firm-specific unsystematic risk and market-related systematic risk. Unsystematic risk is the firm-specific portion of return variability that complementary holdings can offset as the portfolio grows. Systematic risk is the market-related portion that remains after names are combined and that cannot be removed by holding more uncorrelated securities.
Under the CAPM framing, the relevant risk of a security is that systematic, market-related exposure rather than its standalone total variability.
How adding names leaves a market residual
Adding securities reduces total portfolio variability until a market-related residual remains. The archive describes that decline as leveling off after roughly 10 to 35 names.
Unsystematic risk is described as minimized in portfolios of about 35 names, plus or minus 5, drawn from industries that are not closely related. A practical rule of thumb of 20 to 25 names is also offered. Diversification, in this framing, means adding securities from less related industries so complementary return swings cancel firm-specific risk and leave mostly market-related residual risk.
The source states that unsystematic risk can be reduced by diversification and does not raise a security's expected return, because only systematic risk is priced.
What the CAPM prices
Beta is a scaling of how strongly a security or portfolio moves with the market. After diversification has reduced firm-specific noise, beta is the remaining risk that still commands a premium.
Expected return is written as the risk-free rate plus a risk premium. The CAPM states that premium as beta times the market excess return. The security market line is the CAPM risk-return schedule in which the premium on a stock or portfolio varies directly with beta.
A five-name classroom comparison
A five-month student exercise compared two five-name, five-industry books. The higher-beta book, with betas from 0.8 to 1.6, lost 38 percent. The lower-beta book, with betas from 0.6 to 1.3, lost 17 percent. Other variables were held constant.
Even with only five securities, the lower-beta book showed a smaller loss. The source treats that result as minimized risk rather than proof that unsystematic risk had been fully removed.
All readings on this track · 6 readings
- 1990Implied economic forecasts from factor portfolio construction
- 1994Alpha screening and regime-aware portfolio construction
- 2007Sizing a trend sleeve with a risk-aversion sweep
- 2010Portfolio construction as a pipeline: a 2010 classroom case
- 2013Classroom books, leftover beta, and diversification
- 2015Two layers for a managed-futures sleeve in a stock-bond mix