Den här studien undersöker hur vinstvarningar påverkar stora och små företag. För att förklara dess påverkan på företagen har den effektiva marknadshypotesen och behavioral finance använts som grund. Avgränsningen har gjorts till Stockholmsbörsen då inga tidigare studier haft fokus på den. Empirin visar att företag i Large Cap påverkas med i snitt -4,63% och företagen i Small Cap med -8,42%. Large Cap visade signifikanta abnorma avkastningar under eventdatumet och dagen efter medan Small Cap endast visade signifikans under eventdatumet. Vid en portföljjämförelse mellan de två listorna ligger resultatet i linje med den effektiva marknadsteorin. Däremot vid detaljerade mappningar av företagens aktier kan anomalier hittas som kan förklaras av olika psykologiska fenomen inom behavioral finance. / This paper studies how profit warnings affect largeand small companies. The efficient market hypothesis and behavioral finance was used in orderto explain the affect of the profit warnings on the companies’ stocks. The boundary wasdetermined to be the Stockholm OMX since no previous studies had been performed in this particular fashion. The data demostrates that companies within Large Cap are affected with an average of -4,63% and the companies in Small Cap with -8,42%. Large Cap showed significantabnormal returns during the event date and the day after while Small Cap only showed significance during the event date. A portfolio comparison between the two lists reveals results that are in line with the efficient market hypothesis. However when using detailed data of thecompanies stocks some anomalies can be found, which can be explained by psychological phenomena within behavioral finance.

The aim of this study is to investigate if individuals reacts rational to the announcement of negative profit warnings in the Swedish stock market. This is done by using an event study approach, investigating the corresponding abnormal returns and cumulative abnormal returns before, during, and after the announcement. Tests is also made to see whether qualitative and quantitative profit warnings and firm size has any impact on the cumulative abnormal returns. The sample consists of 176 profit warnings from 2008 to 2018. On the announcement day, the average abnormal return at day zero was -6.99 % and the average cumulative abnormal returns at day zero and one was -9.06 %. The results found also that smaller firms generate lower abnormal returns on the announcement date, but that there is no difference between qualitative and quantitative profit warnings. With small and insignificant cumulative abnormal returns before and after the announcement, the reached conclusion is that the market is efficient on aggregate level during the event of negative profit warnings.

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