Decomposing Mutual Fund Alpha into Stock Selection and Weighting

Summary

This research summary explains a method for separating a mutual fund’s alpha into stock-selection alpha and weighting alpha. It estimates selection skill by reweighting a fund’s holdings by market capitalization, then defines weighting alpha as the difference between the fund portfolio’s total alpha and the alpha of that reweighted portfolio. The study uses holdings and return data for active US equity funds and evaluates performance with a four-factor model, along with alternative alpha estimates and controls for measures of active management.

The reported findings suggest weighting contributed more to average alpha and persisted longer than selection: weighting alpha lasted up to 12 months, while selection alpha lasted one month. Funds ranking highly on both dimensions had stronger subsequent alpha in the reported portfolio comparisons. The study also reports that these signals added information beyond conventional activity measures and that results held under an alternative beta calculation. These are historical findings from a defined sample; the summary notes exclusions including fees, trading effects, non-US stocks, and within-quarter portfolio adjustments, limiting direct conclusions about investor returns or current fund performance.

Key ideas

• The method separates selection alpha from weighting alpha by comparing actual holdings with a market-cap-weighted version.
• The study estimates alpha with a four-factor model and tests alternative specifications.
• Reported weighting alpha was larger on average and persisted longer than selection alpha.
• Funds strong in both selection and weighting showed higher subsequent alpha in the study’s comparisons.
• The analysis excludes fees, trading effects, non-US stocks, and within-quarter portfolio changes.

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This summary was written by Stratmill's research agent from the original; it is not a copy of the source.