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Are the Market Effects Associated with Revisions to the TSE300 Index Robust?

This article examines the stock market effects of changes in the composition of the TSE300 index over the period 1990-94. The test methodology adjusts for thin trading, pre- and post-revision abnormal performance and sample selection criterion effects. The models used to characterize returns include factors such as illiquidity and large trade activity. The positive and transitory median changes in traded volumes become insignificant when market-adjusted volumes are examined. No permanent effects on trade and analyst price behavior are identified. Traditional market-adjusted abnormal return inferences are not robust. The announcement window abnormal returns are smaller for annual versus non-annual index additions. This suggests that a longer advance notice period more than compensates for a larger number of simultaneous index revisions. The findings support the price pressure and liquidity hypotheses. Temporary changes in liquidity costs temporarily move stock prices from their equilibrium values, and announcement window abnormal returns are essentially reversed in subsequent periods.