Simpler Better Market Betas

62 Pages Posted: 2 Aug 2019

See all articles by Ivo Welch

Ivo Welch

University of California, Los Angeles (UCLA); National Bureau of Economic Research (NBER)

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Date Written: July 2019


This paper proposes a robust one-pass estimator that is easy to code: Justified by the market-model itself and using a prior that market-betas should not be less than –2 and more than +4, the market-model is run on daily stock rates of return that have first been winsorized at –2 and +4 times the contemporaneous market rate of return. The resulting “slope-winsorized” estimates outperform (all) other known estimators in predicting the future OLS market-beta (on R2 metrics). Adding reasonable age decay, suggesting a half-life of about 3 to 5 months, to observations entering the market-model further improves it. The estimates outpredict the Vasicek estimates by about half as much as the Vasicek estimates outpredict the OLS estimates.

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Suggested Citation

Welch, Ivo, Simpler Better Market Betas (July 2019). NBER Working Paper No. w26105, Available at SSRN:

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