The impact of some expected and unexpected events on markets

This study aims to determine whether the credit ratings given to Turkey by international credit rating agencies, Turkish Central Bank's policy interest rate decisions, terrorist attacks against civilians, and certain political events influence the stock prices of companies in the Borsa İstanbul 30 (BIST30) Index in order to reveal whether the stock market could be considered efficient in the face of the aforementioned events. To this end, the effects of 48 events that occurred between 2008 and 2020 were analyzed through the event study methodology by considering confounding factors. The analyses revealed that the event that had the greatest impact on stock markets was the policy interest rate decisions of the Turkish Central Bank, followed by political events, credit rating scores of international credit rating agencies, and terrorist attacks against civilians, respectively. These findings show that companies in the BIST30 index are not effective in the semi-strong form. The market response to the Turkish Central Bank interest rate decisions can be explained by spurious herding. In contrast, the market responses to political events, credit rating scores are given by international credit rating agencies, and terrorist attacks against civilians are consistent with the uncertain information hypothesis, prospect theory, and adaptive market hypothesis, respectively.