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The distributions of stock returns and capital asset pricing model (CAPM) regression residuals are typically characterized by skewness and kurtosis. We apply four flexible probability density functions (pdfs) to model possible skewness and kurtosis in estimating the parameters of the CAPM and compare the corresponding estimates with ordinary least squares (OLS) and other symmetric distribution estimates. Estimation using the flexible pdfs provides more efficient results than OLS when the errors are non-normal and similar results when the errors are normal. Large estimation differences correspond to clear departures from normality. Our results show that OLS is not the best estimator of betas using this type of data. Our results suggest that the use of OLS CAPM betas may lead to erroneous estimates of the cost of capital for public utility stocks.Robust estimation, Beta, Flexible distributions, Skewness, Kurtosis,

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Robust estimation techniques based on symmetric probability distributions are often substituted for OLS to obtain efficient regression parameters with thick-tail distributed data. The empirical, simulation and theoretical results in this paper show that with skewed distributed data, symmetric robust estimation techniques produce biased regression intercepts. This paper evaluates robust methods in estimating the capital asset pricing model and shows skewed stock returns data used with symmetric robust estimation techniques produce biased alphas. The results support the recommendation that robust estimation using the skewed generalized T family of distributions may be used to obtain more efficient and unbiased estimates with skewness.

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