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All Events Induce Variance: Analyzing Abnormal Returns When Effects Vary across Firms

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  • Harrington, Scott E.
  • Shrider, David G.

Abstract

We demonstrate analytically that cross-sectional variation in the effects of events, i.e., in true abnormal returns, necessarily produces event-induced variance increases, biasing popular tests for mean abnormal returns in short-horizon event studies. We show that unexplained cross-sectional variation in true abnormal returns plausibly produces nonproportional heteroskedasticity in cross-sectional regressions, biasing coefficient standard errors for both ordinary and weighted least squares. Simulations highlight the resulting biases, the necessity of using tests robust to cross-sectional variation, and the power of robust tests, including regression-based tests for nonzero mean abnormal returns, which may increase power by conditioning on relevant explanatory variables.

Suggested Citation

  • Harrington, Scott E. & Shrider, David G., 2007. "All Events Induce Variance: Analyzing Abnormal Returns When Effects Vary across Firms," Journal of Financial and Quantitative Analysis, Cambridge University Press, vol. 42(1), pages 229-256, March.
  • Handle: RePEc:cup:jfinqa:v:42:y:2007:i:01:p:229-256_00
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