While the statistical explanation of the R squared is that it provides a measure of the
goodness of fit of the regression, the financial rationale for the R squared is that it
provides an estimate of the proportion of the risk (variance) of a firm that can be
attributed to market risk; the balance (1 - R
2
) can then be attributed to firm-specific risk.
The final statistic worth noting is the standard error of the beta estimate. The
slope of the regression, like any statistical estimate, is made with noise, and the standard
error reveals just how noisy the estimate is. The standard error can also be used to arrive
at confidence intervals for the “true” beta value from the slope estimate.
Estimation Issues
There are three decisions the analyst must make in setting up the regression
described above. The first concerns the length of the estimation period. The trade-off is
simple: A longer estimation period provides more data, but the firm itself might have
changed in its risk characteristics over the time period. Disney and Deutsche Bank have
changed substantially in terms of both business mix and financial leverage over the last
few years and any regression that we run using historical data will be affected by these
changes.
The second estimation issue relates to the return interval. Returns on stocks are
available on an annual, monthly, weekly, daily and even on an intra-day basis. Using
daily or intra-day returns will increase the number of observations in the regression, but it
exposes the estimation process to a significant bias in beta estimates related to non-
trading.
23
For instance, the betas estimated for small firms, which are more likely to
suffer from non-trading, are biased downwards when daily returns are used. Using
weekly or monthly returns can reduce the non-trading bias significantly.
24
The third estimation issue relates to the choice of a market index to be used in the
regression. The standard practice used by most beta estimation services is to estimate the
betas of a company relative to the index of the market in which its stock trades. Thus, the
betas of German stocks are estimated relative to the Frankfurt DAX, British stocks
23
The non-trading bias arises because the returns in non-trading periods is zero (even though the market
may have moved up or down significantly in those periods). Using these non-trading period returns in the
regression will reduce the correlation between stock returns and market returns and the beta of the stock.
24
The bias can also be reduced using statistical techniques suggested by Dimson and Scholes-Williams.