Resumen
This study explored the price reactions of shares moving in and out of Johannesburg Stock Exchange (JSE) Top 40 Index by applying three models to calculate the abnormal returns of the stocks; namely: the market model, the Capital Asset Pricing Model (CAPM) using leveraged betas and CAPM using unleveraged betas. An event study methodology was used to measure the abnormal returns around the event date windows. The average abnormal returns (AARs) and cumulative average abnormal returns (CAARs) were tested for statistical significance for various windows. The three models used show increasing (decreasing) CAARs for additions (deletions) when examined over the longer-term windows; however, CAARs showed mixed results for short-term cumulative windows. The study found price reversal for stock prices over the longer-term horizon for prior and post events. The stock price AARs for the pre-announcement date were found to be significant for the stocks to be deleted from the index, but not statistically significant for additions. There were higher asymmetric abnormal average returns for the deletions and additions when using the market model approach. The study further found that the deletion from the index and addition to the index experienced positive and negative AARs respectively on a change day of the event. This implies that it is possible to earn average abnormal returns. However, such anomalies should disappear if the market becomes more efficient.