Long-horizon event studies and event firm portfolio weights: Evidence from U.K. rights issues re-visited

Empirical results, in long-horizon event studies, are sensitive to whether equal- or value-weighting schemes are used to form event firm portfolios. In this paper we propose, as a first step, an evaluation of the economic value to investing in an equal-weighted and a value-weighted event firm portfolio prior to the event study. Using tests for mean-variance spanning we find in our data a significant improvement in an investor's investment opportunity set to investing in an equal-weighted portfolio. We then re-visit the long-run post-offer performance for rights issuers in the U.K. where we find, as in other studies, differences in both the magnitude and statistical significance of abnormal performance for value-weighted and equal-weighted event firm portfolios. We then use the results from our first step to provide an economic rationale for interpreting our results. A general conclusion we draw is that it may be useful to first ascertain, in long-horizon event studies, the economic value of portfolio weighting schemes as this can then provide some guidance to the use of a specific portfolio weighting scheme and thereby to interpretation of often conflicting results.