The influence of rumors in the stock market: a case study with Petrobras

This paper analyzes the influence of rumors on price fluctuations in the Stock Exchange of Sao Paulo between 2007 and 2011, through a case study with Petrobras, a company whose stock had the largest trading volume within the period. For this purpose we used the historical prices of cash market provided by the stock exchange. The communications in which Petrobras provides clarifications regarding unofficial information disclosed in the press were also collected from the stock exchange website. The analysis of these documents helped to create a diagram to represent the information about the rumors and categorize them by subject. This diagram was applied to a database to store the information collected from the company's communications. Then this information was retrieved to analyze the influence of rumors on price movements. The results confirm that the company's responses to rumors influence price fluctuations of its stock. At eagerness for information to dilute uncertainty, investors make decisions based on rumors betting on the credibility of the media that disclose them, even though knowing that the information is not always reliable.